Provided is a power supply device including: a magneto generator (1), which includes: a rotor including a magnet forming a magnetic field; and a stator which generates an alternating current in stator windings by rotation of the rotor; a rectifying unit (3) which rectifies the alternating current generated by the magneto generator to a direct current; a variable transformation-ratio direct current voltage transformer (40) which transforms an output voltage of the direct current of the rectifying unit to a voltage between input terminals of an electrical load (2) to which electric power is supplied; and a voltage control unit (5) which controls a transformation ratio of the variable transformation-ratio direct current voltage transformer in accordance with at least one of an operating state signal regarding the rotation of the rotor of the magneto generator and an electrical load state signal of the electrical load.