The present invention provides a control power supply capable of stably operating to output a desired voltage as electric power, even when the input voltage thereof varies over a wide range.A power supply includes a rectification circuit 2 for rectifying the AC output from a power generator 1 and a non-insulation type DC/DC converter 3 for stepping down the DC output from the rectification circuit 2. A self-excited oscillation type converter (RCC) 4 is provided at the following stage of the non-insulation type DC/DC converter 3. The input voltage which has been stepped down by the converter 3 is input to the primary side of the RCC 4, and the RCC 4 stably operates with the input voltage which varies largely to supply, from its secondary side, a power supply to an ECU 5 or the like.