Looking at the other files in the collection, it would seem that they were all intended to be licensed under GPLv2, but the author was a bit lackadaisical with his license text. One file includes some standard license boilerplate with a warranty disclaimer, another includes an abbreviated note, several include very vague mentions of being "free" without any elaboration and those two lack any legal text whatsoever.

If you're concerned but really want to use it, you could probably get anybody whose Japanese is less rusty than mine to email Masanao Izumo and ask him to confirm that he's offering it under a free license.