A power supply device includes: a magneto generator with a rotor having a magnet; a torque supplying device supplying torque to the rotor; a rectifier circuit supplying electric power to a electrical load device by rectifying an output of the magneto generator; a short circuit electrically shorting an output end of the magneto generator; a voltage detection circuit detecting a terminal voltage of the electrical load device; and a control circuit controlling a voltage of the electrical load device to be a first set value by controlling the short circuit to switch ON (short operating mode) and OFF (rectification operating mode) according to the voltage detected by the voltage detection circuit and allowing the short circuit to operate while making a switching between the two operating modes according to a operating state relating to a rotation of the rotor of the magneto generator.