PURPOSE: To ensure protection, and to improve accuracy by mounting an overcurrent detecting circuit detecting oscillating currents in an oscillation circuit at all times and a constraint circuit ending oscillating currents by an overcurrent signal from the detecting circuit.

CONSTITUTION: An inverter circuit is constituted of an oscillation circuit 10, to a capacitor C thereof load 30 is juxtaposed, switch circuit elements 21, 22 and a changeover command circuit 60 alternately turning the elements 21, 22 ON-OFF, and supplies the load 30 with AC power. An overcurrent detecting circuit 40 and a constraint circuit 50 are mounted at that time. When the overcurrent detecting circuit 40 detects excess oscillating currents, a switch circuit element 20 is constrained under the state of either of ON or OFF by the constraint circuit 50 by an overcurrent signal from the circuit 40, and excess oscillating currents are ended. Accordingly, the generation of excess voltage in a reactor L in the oscillating circuit 10 is relaxed, thus improving protective performance.