To provide a rotational position detecting device in which a rotational position detection error caused by variation in distance between a sensor magnet and a magnetic sensor is reduced, the rotational position detecting device includes a sensor magnet (30) configured to generate a magnetic flux; a holder (34), which is configured to hold the sensor magnet, and has a pressure-contact portion (340) to be fixed to a shaft (40) of a motor (1); and a magnetic sensor (32) configured to detect the magnetic flux generated by the sensor magnet, the magnetic sensor being directly or indirectly fixed to the motor, and being arranged away from the sensor magnet in an axial direction of the shaft, the shaft having an abutment portion (403, 405) configured to receive a distal end portion (341) of the pressure-contact portion.