PURPOSE: To settle each output voltage at a predetermined level without causing useless power consumption by performing switching control of the joint between a ground line and the secondary winding of a transformer so that positive and negative output voltages have predetermined levels when the load varies.

CONSTITUTION: Since a switching control section 11c closes corresponding one of analog switches 11d connected with the secondary winding N2 of a transformer based on a digital signal representing the shift of output voltage, the secondary winding N2 of the transformer is divided at a winding ratio corresponding with the imbalance of load with respect to the closing point of contact. Consequently, the number of secondary winding increases automatically on the positive side with respect to a ground line LG when the load current is higher on the positive side than on the negative side. When the output current is higher on the negative side than on the positive side, one of a group of analog switches lid is closed so that the number of secondary winding increases automatically on the negative side. According to the invention, output voltage can be kept constant even if the load varies irregularly.