Well, my Japanese sucks and so does Google Translate.

But as far as I understood the license it's free to use. There's also an implementation for game maker. On the forums it says: "I believe Pixel asserts that there is no limitation on pxtone's use but asks politely to attribute him for his work. To do so he asks for you to use the banner he provides, pxtone.bmp, in anyway you like."

(Edit: In the readme I have it doesn't really say anything about a banner, although from what all these different sources say it's pretty clear that the software is free to use and the dev would appreciate being mentioned somewhere if possible)