Abstract: The invention relates to a sensor provided with a magnet (52) and a magnetic detecting element (53) comprising a detection shaft (38a), a detection disc (59a) which is fixed to the detection shaft, a magnetic piece (59b) which is fixed to the outer circumference edge of the detection disc, a yoke (59c) which is disposed to hold the magnetic piece in a predetermined shaped gap, and the magnet and the magnetic detecting element both fixed to the yoke. The sensor is used for a drive force auxiliary device which combines a human power drive force and a motor drive force. The drive force auxiliary device comprises a torque sensor (30), a vehicle speed detecting means (40) for detecting a running speed of the drive force auxiliary device, and an auxiliary power means using a motor (M). The vehicle speed detecting means (40) detects a running speed by virtue of the drive force auxiliary device based on a rotation position signal obtained from a rotor rotation position detector.