PURPOSE:To enable proper use to be attained in a single set of device, when it is in normal action and in standby action, by means of a selector switch so as to select fed back voltage. CONSTITUTION:The AC power of an AC power source 1 is converted into DC power, the DC power is converted to AC power by a switching element 19, this AC power is transformed by a transformer 16, the output of the transformer 16 is rectified by a rectifying circuit comprising diodes 22, 27, 29, 32, and the rectified DC power is outputted. Of the voltages at points alpha and beta, a higher one is selected by diodes 34, 35, smoothed and compared with the reference value in a voltage comparator part 14. The switching element 19 is controlled through a PWM circuit 15 so as to obtain agreement with the reference value. When a transistor 40 is turned on, voltage division ratio becomes small, the output voltage, decreases and the alpha side voltage having non-linearity is selected. When the transistor 40 is turned off, the voltage division ratio increases, the output voltage increases, and the beta side voltage having linearity is selected.