A small and light weight DC/DC converter is provided comprising a coil in which a first coil and a second coil are wound in opposite directions (reversed phases) on the same core is used, and at the same time as when an excitation current is flowed in the first coil with a switch ON and a switch OFF, an electric current is generated in the second coil in a direction to offset the magnetization of the core, and the electric current from the second coil is transferred to an output side capacitor. Similarly, at the same time as an excitation current is flowed in the second coil with the switch ON and the switch OFF, an electric current is generated in the first coil in a direction to offset the magnetization of the core, and the electric current from the first coil is transferred to the output side capacitor.