There has been some discussion in the Visa eligibility section about whether you are allowed to work if you have a dependent visa. The answer is that you can, but only if you apply for permission and get a work permit.
It’s a bit confusing, because the Ministry of Foreign Affaris website (under Types and Categories of Visa) says that you may not work on a dependent visa.
Under the “Statuses of residence for engaging in specified activities in Japan”, there are three categories:
(a) Statuses of residence permitting work
(b) Statuses of residence not permitting work
(c) Whether work is permitted or not depends on the contents of individual permits
Dependent visa is listed in the category of “(b) Statuses of residence not permitting work”. This seems pretty clear-cut — you can’t work if you are a dependent. The activities you may carry out are “Daily living activities on the part of the spouse or unmarried minor child of those who stay in Japan with a status of residence mentioned in Table (a) (excluding Diplomat and Official) or with a Cultural Activities or College Student status of residence.”
However, a few people have commented (special thanks to Bryet and Mark) that it is possible to get a work permit, which allows you to work with restrictions (e.g. the type of job or the hours you can work). You should apply to the immigration department if you need more information on this.