It is illegal for non-Japanese to work in Japan without a proper visa. Holders of student or cultural visas who have filed for permission to work, or holders of working-holiday visas, can work legally in Japan under certain restrictions. A full-time job requires an employer-sponsored working visa. There are legal employment categories for foreigners that specify standards of experience and qualifications.
The most common job is English-teaching; however, it is increasingly difficult to find jobs while in Japan that offer visa sponsorship. If you do find one, know that the sponsorship process can be a lengthy one – typically taking at least three months.
The first step is to apply for a Certificate of Eligibility, which requires handing over any number of documents (depending on the desired visa status), at the nearest Japanese immigration office. Once this certificate has been issued, you can then apply for a visa. Some companies may handle some or all of this process for you.
Given the high cost of living in Japan, it makes sense to secure employment and a working visa while still in your home country or while in Japan on a visa that allows part-time work (such as a student or working-holiday visa). The Japan Exchange and Teaching (JET) Programme (jetprogramme.org), as well as some large English conversation school chains, sponsor qualified applicants.