1st, Only a few places in Japan was in ruins after WWII and

retained some of its war making capabilities (Japan said if MacArthur wanted, Japan could immediately remake 2 million troops with equipments to join the Korean War is a prime example of really how much infrastructures Japan had lost); 2nd, Japan had robbed and hoarded much wealth from Asian countries during the war and were most likely using that to rebuild; 3rd, Japan didn't pay huge war reparations to victim countries instead it LOAN money to them at little interest (still making money even with LITTLE interest)

All factors concerned, without an active military of its own, it was easy for Japan to live under US protection using the pretense of anti-communism while solely developing its post war economy for 1/4 of a century using the wealth it had taken during the war