PURPOSE: To provide a power converter having a high efficiency and a low generating cost by always monitoring a DC voltage and an AC voltage, and switching to a tap of a transformer having a high efficiency to operate it.

CONSTITUTION: A calculator 9 monitors a solar cell voltage Vd and an AC power source voltage Va outputs a current reference I and outputs a command (s) for turning ON/OFF a contact 12a from correlation between the Vd and the Va. A driver 14 excites an operation coil 12b according to the command (s) to open or close the contact 12a. When the varying range of the Va is V1--V3 and the varying range of the Vd is Vd1-Vd4, the calculator 9 decides an operating area from the relation between the Va and the Vd, and operates to close the contact 12a in the case of the area of A-B-C and to open the contact 12a in the case of the area of B-C-D-E. Since the contact 12a is closed in the case of the range impossible to be controlled and opened in most time for use by reducing a transformation ratio, circuit currents of a primary side of a transformer and an inverter bridge 2 are small, and an operation of high efficiency can be performed.