I always thought that it was Masiya, but pcecp.com says that it's Masaya.
On a side note: I checked my Japanese Double Dungeons manual for a correct spelling, but no dice (no English); when I checked my US version, I found that the manual is in French. I bought it a couple of years back from TZD, but apparently never opened the manual; now I wonder if I have any more Canuck versions.