PURPOSE: To suppress the oscillating operation of a switching device by supplying an auxiliary voltage signal to comparing and driving circuits from a point of time when a control signal is changed up to a point of time exceeding the switching of an operation state.

CONSTITUTION: A switching circuit 40 selects one of high level voltage VH and low level voltage VL in accordance with a control signal from a driving circuit 42 and outputs output voltage. A 1st state judging circuit 56 detects a change in a control signal from the circuit 42 to the 1st level and a 1st voltage impressing circuit 60 impresses voltage capable of setting up an input from a comparator 54 to a level larger than an upper threshold level to an adder 52 for a fixed period in response to a detection signal from the circuit 56. A 2nd state judging circuit 58 detects the control signal from the circuit 42 to the 2nd level and a 2nd voltage impressing circuit 62 impresses voltage capable of reducing the input voltage of the comparator 54 to a level smaller than a lower threshold level to the adder 52 for a fixed period in response to the detection signal.