An actuator is configured to drive a boost pressure control valve of a supercharger and includes an electric motor, an output shaft, a speed reducer, a rotational angle sensor and a magnetic circuit holder member. The speed reducer includes a final gear. The final gear is made of metal and is fixed to the output shaft, and the speed reducer reduces a speed of rotation outputted from the electric motor and transmits the rotation of the reduced speed to the output shaft. The rotational angle sensor includes a magnetic circuit device and a sensing device and senses a rotational angle of the output shaft. The magnetic circuit holder member is a non-magnetic member fixed to the output shaft. The magnetic circuit holder member is formed separately from the final gear and holds the magnetic circuit device.