On Super Nintendo games, there are two notches cut out on the backside of every game. They fit around two pegs that are inside the slot where you insert the cart into your system. Super Famicom carts do not have these holes on them, and this is the only thing keeping you from playing Japanese games. Modifying your Super Nintendo is actually quite simple and very cheap (or free if you already have everything). You can still play your US carts just fine, so there is no downside to this. There are a few ways to play Japanese games, but this instructable will guide you through modifying the actual console itself by removing those two pegs.
Note : Whenever you work with anything, there's a chance you may break it. Be careful.
Step 1: Tools Needed
Flat, clean work surface
4.5mm Security Bit + Screwdriver handle
Small Philips Screwdriver
Xacto Knife with sharp blade
Small metal file or low grade sandpaper
Small lid or container (to put screws into)
Push pin or Needle
Q-Tips and Water
The only thing you probably do not already have access to is the Nintendo Security Bit for their special screws. You can easily find it on EBay (paired with it's 3.8mm counterpart for opening up the carts) between $5-10. If you need it, click HERE and order yourself up some.
I've read and tried some other methods for removing the security screws but I couldn't get them to work myself. Do a quick google search for alternate methods if you don't feel like paying for the bit.