How to JailBreak the Samsung Galaxy S2 sc-02c Docomo Japan

I assume you have a stock SC-02C by docomo. If you don't, download the firmware from sammobile and flash with ODIN.

Then flash Siyah Kernel 5.0.1 for S2 with ODIN:
http://d-h.st/Bcf

Hold Home+Volume Up+Power to go into recovery mode. Wipe Data/System.
Reboot into your rooted SC-02c.

If you need to break the sim lock of the phone copy this apk to the phone and install it. It will remove the sim lock:

http://d-h.st/6EfJ

After you have ensured SU exists and have used some manager to remove the junk apps by docomo re-flash the original kernel by extracting the zImage from the samsung firmware tar file and flashing the kernel only (careful don't reflash the entire firmware because you'll end up doing everything from scratch).

Re-flashing the original kernel is important to get the GPS working, I couldn't get WjChen0's gps fix to work (by adding it under /etc/init.d)

Good luck.