With the earliest films being produced in the early 1950s, that would have been less than a decade after defeat so it seems Japan's military would still be depleted. During the 60's the Japanese military was depicted even more advanced with space-aged ships and experimental weapons. I know these movies are comedic but as a child seeing the military operate in these films always astounded me and lead me to believe they were the most advanced fighting force on the planet. What was the real state of the Japanese military during the era of this films though. Also what was up with their fascination of giant robots? Did people at time really think giant mechs would be the future of war?
Immediately after WWII, Japan was not permitted to have a military. However, only a few short years later large elements of the American occupation forces were transferred from Japan to Korea. A disarmed Japan with little or no ability to defend itself was desirable to neither the Japanese or the American leadership, and so the pragmatic decision was made - Japan would rearm, at least in part.
Starting in 1950, a reserve force was raised to an approximate strength of a few divisions (75k personnel). In theory, it was an extension of the police forces. By 1954 this had swollen to over a hundred thousand and included some maritime elements and (on paper) an airforce. This was the year that it was officially reorganized into the Japan Self Defense Force and its constituent branches.
1954 was also the year Godzilla came out. It's safe to say that in 1954, the newly minted JSDF was not an unusually advanced military force, nor an especially large one. In the 1960s, the JSDF certainly did not have rayguns and advanced experimental technology, that sort of thing is purely an invention of the fiction as you speculate. The Japanese military industry did develop a lot of effective indigenous designs, from the Cold War right through to today, but it was generally the United States and Soviet Union that pushed boundaries of military technology.