you are in IT or techie stuff?? then there are lots of possibilities for you
but if you graduate in your country and come to Japan as a fresh graduate, oppurtunity is kinda slim. But if you study and graduate in a top level university in Japan, then your chances is big.
If you are a student here, chances are just like what MMM said, teaching is a good way to earn money. Most students I know, who have permission from their respective universities of course, teach/tutor English during their free time. It's basically the easiest way. Other part-time jobs are kinda hard on their school schedule, and teaching is very flexible.