Although this is a broad topic and the issue of technological comparison is much harder to quantify that it might look, in general the IJN started the war at a roughly comparable level of technological sophistication, but this rough parity was eroding even by the onset of the Pacific War.

The Japanese investment in naval armaments in the interwar period had meant that a number of its units possessed equipment that was thoroughly tested and integrated into existing doctrine. The Fubuki-class destroyers were a case in point in that they pioneered elements of destroyer development (heavy DP-gun armament, torpedoes, high-speed, good optics) that served Japan well in the opening stages of the war. Japanese torpedoes, unlike their American counterparts, had a thorough testing regime which meant that not only were IJN captains used to firing them, their torpedoes would actually detonate when hit. Although Japanese ships tended to be top-heavy and over displacement, they were highly capable units. For the type of roles and warfare of the 1930s and early 1940s, the IJN was a relatively balanced force and more than capable of performing the missions assigned to it. The IJN also pioneered a number of innovative technologies and ship designs, such as a dedicated landing craft, heavy destroyers, and midget submarines. Although some of these innovations did not pan out, most notably midget submarines, IJN innovation tended to pioneer elements that became standard for navies in the Second World War.

Th problem for the IJN, and Japan as a whole, was that the scale and pace of technological development engendered by the war far outstripped Japan's own comparatively meagre industrial base. The result was that Japanese forces had to make do with preexisting weaponry that was fast becoming obsolescent compared to their American rivals. For example, both the 12.7 cm/40 Type 89 naval gun and the 12.7 cm/50 Type 3 naval gun were an excellent DP guns for 1930s aircraft, but the growth in speed of aircraft meant that their mountings and slow rate of fire could not handle the new generation of American aircraft. The primary light AA, the Type 96 25 mm, was again a decent weapon for the 1930s-generation of aircraft, but was patently inadequate for meeting the aircraft of the 1940s. For their part, the Japanese were aware of these shortcomings. The postwar USN technical mission to Japan found in interviews with IJN officers, that the Type 96's power and manual mountings were inadequate, the sights ill-adapted for high-speed targets, and that rates of fire, ammunition supply, and muzzle blast were poor. For larger guns, Japanese mountings were heavier and manpower intensive than USN counterparts, which were experimenting with highpowered servo mechanisms and other semi-automated features that made these guns more efficient. But the IJN simply had little alternative but to make do with its stock of weapons and their designs. The IJN was able to field a few replacements for this outdated equipment, such as the excellent 10 cm/65 Type 98 naval gun, but for the most part these replacements suffered from both issues of quantity and poor quality control.

In a number of areas of maritime technology, the Japanese had clearly fallen behind the US by Pearl Harbor and Japan never was able to achieve anything approaching parity. In terms of machinery, the US Naval Technical Mission reported in December 1945 that

there were no really new design features in the main machinery plants of major Japanese combat surface vessels since the late thirties.

Aside form the experimental Shimakaze one-off, the IJN did not adapt high-pressure steam boilers and IJN machinery remained bulky and relatively heavy compared to their American counterparts. Prior to Pearl Harbor, the German Naval Attache to Japan, Paul Wenneker, often noted that Japanese ship welding techniques were sub-par compared to Western states and the US Naval Technical Mission confirmed this assessment, noting that both workmanship and quality control on Japanese welds was much poorer than US standards. In crucial non-optical sensors, the Japanese were likewise behind the USN by Pearl Harbor. IJN radar and hydrophones were also uncompetitive designs. The main Japanese wartime hydrophones, the Type 93/Type 0, were based upon imported US MV-hydrophones bought in the early 1930s. The range of the hydrophones was relatively poor and suffered from quality control issues. By the same token, Japanese sonar development was also relatively slow-paced by comparison to its British and American opponents. The IJN did recognize the importance of radar quite early with some of the first sets fitted to ships in 1942, but the limitations of the Japanese electronics industry meant that IJN radar sets operated under the double burden of both being too few in number and less capable than their opponents' equipment. IJN explosives likewise remained stuck in the 1930s; there was no developmental equivalent to Torpex in the IJN's arsenal. While the yawning technology gap was quite serious on its own, the IJN was also facing an increasing quantitative gap with its USN opponent. In December 1941, the USN had eight battleships and five Essex-class carriers on the slipways because of the Two-Ocean Navy Act. Japanese shipyards could not keep with this pace of construction. American quantitative superiority often dovetailed with its own technological superiority. For example, the Mark 12 5"/38 caliber gun was the main intermediate gun of the USN and was already a decent gun by interwar standards. The wartime development of technology like proximity fuses, radar-directed fire, highpowered servo mechanisms made an already capable gun much more deadly.

The irony in all this was that the interwar IJN had counted upon technology to be a trump card in its expected battle with the Americans. For the 1930s, the IJN was arguably ahead of the USN in a number of key areas and retained a lead in some areas like torpedoes, through 1943. The problem was that Japanese industry could not keep up with the pace of technological development. Japanese research institutions, while capable of producing some innovations, were comparatively small and not able to affect the Japanese war effort as a whole. Endemic competition between the IJN and the IJA also further exacerbated attempts to harness Japanese science to the war effort more efficiently. Their American opponents had the luxury of large research institutions, bountiful natural resources to apply innovations, an a large industrial base to overwhelm the Japanese with both quantitatively and qualitatively superior equipment.

Sources

Evans, David C., and Mark R. Peattie. Kaigun: Strategy, Tactics, and Technology in the Imperial Japanese Navy, 1887-1941 Annapolis: USNI Press, 1997.

US Naval Technical Mission to Japan- digitized here: http://www.fischer-tropsch.org/primary_documents/gvt_reports/USNAVY/USNTMJ%20Reports/USNTMJ_toc.htm