deportation? banned from the country? difficult to enter/work in other countries, i.e America? fine?
1. Deportation - possible
2. Banned from the country - probable
3. Go to jail - possible
4. Difficulty going to other countries like the USA - YES. Because when you apply for US visas they will ask you if you have ever violated immigration laws in the USA or ANY OTHER country. If you write NO and they find out YES, the US immigration people are not idiots, they will find out considering the close working relationship US and Japan have with each other.
In additions, it will ask if you have ever been charged with a crime, illegal immigration IS A CRIME, put NO and they find YES, then you screw yourself even more.
So in the end DON'T DO IT.