I am a student in Japan and I want to visit South Korea in April. What documents do I need for that?
And can Indian passport holders in Japan enter Korea using a transit visa?
A Japanese visa doesn't give you special rights in Korea (unlike, say, a US, Canadian or Australian visa). You are not eligible for the 30-day exemption.
You'll have to apply for a visa in one of the Korean consulates in Japan if you want to visit.