This revision (June 2015) is mostly to say that I've verified the answer by ell. I found the Proceedings volume in question in the U. Tokyo library. It is basically a bound volume of photocopies of handwritten notes in Japanese. I made a "scan" with my phone, so if you want a copy of the Japanese version, just ask. The main theorems are identical, but the arguments in the English version are substantially more detailed, and the list of references was expanded.
Old Partial answer: You can find a copy at one of the libraries listed here. If I don't forget, I can take a look late next month when I go to U. Tokyo for a conference. You can also buy a copy of the proceedings for 4500 Yen (plus shipping).