[nandemo]

> Japanese computer systems are often not using Unicode but are based on other encodings like shift-jis.

That's true but doesn't really answer GP's question. Shift-jis is an encoding for one of the JIS X character sets. Unicode includes all the characters defined in JIS.

http://unicode.org/faq/han_cjk.html#8

Though if we enter into details it gets a bit messy due to the complications due to different simplifications occured in (mainland) China and Japan, questions about different glyphs for the "same" character, etc.