To provide a magnet assembly being simply incorporated into a housing.

The magnet assembly consists of two yoke pieces 1A, 1B, one magnet piece 2 and a box-shaped housing 3, in which the magnet piece and the yoke pieces are housed. A pair of slits 10, into which the two yoke pieces are inserted, are formed to the front wall of the housing while at least one of the front wall or a backwall is opened and an opening 9, into which the magnet pieces is inserted, is formed, and engaging sections 12 engaged with retaining sections 4 formed to the side sections of each yoke piece are formed to the internal surfaces of both side walls while an elastic arm 13 is formed to the internal surface of the backwall at that time. Accordingly, the yoke pieces and the magnet piece can be assembled into the housing extremely simply only by singly inserting one yoke piece into the housing through the slit, putting the magnet piece into the housing from an opening on the front wall or the rear wall side this time, and lastly inserting the residual one yoke piece into the housing the slit.