Abstract: A rotor (17, 31, 71, 91, 101, 121, 131, 141, 201, 401) of a motor (13, 51) is provided with a rotation shaft (21, 59, 410) and a plurality of magnets (15, 53, 75, 105, 210A, 210B, 411) on a circular periphery. Plates (25, 25A, 25B, 25C, 63, 77, 220, 300, 430) made of magnetic materials are provided so that each of which is magnetized by leakage flux of a corresponding magnet (15, 53, 75, 105, 210A, 210B, 401). The plates (25, 25A, 25B, 25C, 33, 63, 77, 107, 220, 300, 430) are disposed along a circular path such that a maximum flux density is formed at both peripheral ends. A magnetic sensor (27) outputs a signal in response to the variation of a flux density on the circular path.