Basically, the yen did experience heavy inflation during the post-WWII occupation years before being stabilized. As a quick history of the leadup to this, the yen was first minted on a silver standard at the beginning of the Meiji era, giving it a value on the same order of magnitude as other western currencies on metal standards. It was later moved to a gold standard in 1897, abandoned this standard during the WWI years, and then briefly moved back onto the gold standard again in 1930 before quickly dropping it again (the timing of this last move had been disastrous, as it was at about the same time the Great Depression was causing many western nations to abandon the gold standard and principles of free trade).
Inflation started to become a problem for the Japanese economy in late 1942, but it didn't really get out of control until the last months of the war, when the military budget was being spent at an even faster than normal rate to prepare for the "decisive battle" with the USA. This did not get better once the war ended - wholesale prices of typical consumer goods in the legal market increased by 539%, 336%, 256%, and 127% in the 1st, 2nd, 3rd, and 4th years of the occupation respectively. To give an idea of the prices this resulted in, 1.4 kg of rice was 2.7 yen in June 1946 but had risen to 62.3 yen in March 1950. Mocking the situation, the comedian Miki Torirō wrote a slapstick song in which someone riding a train finds that the price of oranges has risen between every stop they arrive at. Prices in the black market were even worse, generally many times higher than those on the legal market.
Inflationary trends were eventually brought under control when the Americans dispatched the banker Joseph Dodge in 1949. Dodge was a traditional fiscal conservative, and he curbed the investment loans being made available, drastically reduced government subsidies, and enforced a surplus on the government budget. He also set a fixed exchange rate of 360 yen to the dollar, slightly undervaluing the yen to stimulate Japanese exports. These policies successfully stemmed the rate of inflation, but at the cost of slashed budgets for education and public works, increased unemployment, lower domestic consumption, increased bankruptcy rates in small businesses, and decreased production of durable goods. External observers warned that Japan was on the verge of a depression and that Dodge's policies were akin to economic suicide. However, this possibility was never put to the test, as the outbreak of the Korean War provided a massive stimulus to the Japanese economy with orders for American "special procurements" flooding in. The fixed exchange rate of 360 yen to 1 dollar remained for a while longer until it was lifted by Nixon, at which point the rate immediately arrived at 300 yen to a dollar and eventually peaked at 87 yen to the dollar, close to the modern rate.
Source: Embracing Defeat, by John Dower