To provide an AC - DC electric power supply system and an electric power supply controller capable of achieving cost reduction, high efficiency and easy operation.

By having a controller 11 formed as a linear regulator to reduce the complexity of the controller 11 and a system 10, related cost is related. An active area is formed as a voltage zone, which makes output voltage approach input voltage to provide efficient linear operations of the controller 11 and the system 10. An output transistor is made disabled by a voltage higher than that in the active area, to reduce a voltage drop across the output transistor for further improvement in efficiency.