Ministry of Foreign Affairs
The Visa section on the Japanese Ministry of Foreign Affairs website has all the official information about Japanese immigration, visa requirements and so on. This includes entering Japan, applying for visas and other important topics for foreigners interesting in working in Japan.
The website can sometimes be hard to figure out, because there is a lot of information on all kinds of exotic visa categories and regulation. It is all in English though — very helpful! — with some information also available in other languages.
The rest of the Ministry of Foreign Affairs website shows the Japanese government’s take on foreign affairs and foreign relations. It has a lot of information on Japanese foreign policy, economic affairs and so on.