To provide a dynamo for a bicycle capable of suppressing the weight increase of wheels, preventing the falling of a magnet, and facilitating the manufacturing and machining of the magnet.

A dynamo for a bicycle has a yoke portion 18 rotatable with a wheel 3; and an iron core portion 19 fixed to a hub shaft 15. The iron core portion 19 has first and second iron cores 21 and 22; and a magnet 23 having two poles and a coil 24 which are sandwiched between both iron cores 21 and 22. The first and second iron cores 21 and 22 have a plurality of first and second pole pieces 21a and 22a at predetermined intervals in a circumferential direction. The first and second pole pieces 21a and 22a are opposite to each other while keeping a predetermined gap 29 in a diametrical direction. The yoke portion 18 has a plurality of yoke pole pieces 18a at predetermined intervals in the circumferential direction. The yoke pole pieces 18a can freely enter/leave from spaces between the first pole pieces 21a and the second pole pieces 22a in a rotating direction of the wheel 3.