PURPOSE: To respond to the variation of specification easily as well as to realize a small size and a low cost of the device, by forming a driving circuit and an oscillation circuit with different substartes, and operating the two substrates by a single power supply circuit.

CONSTITUTION: An oscillator 3 is composed of a DC power source 1, an inverter circuit 8 in which FETs 7a to 7d are connected in a full bridge, an oscillation circuit 9 to set the frequency of the circuit 8, an FET driving circuit 10 to drive the circuit 8, a control circuit 11, and the like. By composing the circuit 10 and the circuit 9 with different substrates separately, the conversion can be carried out easily when the output of the inverter device is powered up, for example, because the purpose can be accomplished by only increasing the number of phases in the circuit 10. When such an increase of the number Of phases is made, the substrate is formed by laminating in prural phases through a spacer, and the plural power source circuits are connected to a power source transformer 35, and therefore, only a single transformer can drive the FETs. Consequently, a small size and a low cost of the device can be realized.