Well, to be fair, the OP had already found the music for free, but was concerned about the legality of that which he found.
Now, I haven't looked at the website, nor have I researched the copyright status of the music in question, but even if the game producers have put the song out for free download, there's at least some chance that anyone redistributing it is doing so illegally.
I didn't do a thorough search, so someone else might come up with a more definitive
answer, but it appears that Sen (the creator of Mad Father) allows for free downloads of the
game but there are some restrictions on its use and the use of the music from the game.
Live streaming of the game is prohibited and for-profit uses are prohibited and skipping opening credits of the game is prohibited, among other prohibitions.
The game music, separate from the game, is apparently available under different licenses and for non-commercial use only. While you can listen to the music freely as part of the game, to be legally safe you will want to check out the licensing for each separate song before downloading any of them separate from the game.
