To vary the oscillation frequency of an oscillation circuit, which generates an oscillation signal by inverting the output voltage according to the charging voltage of a capacitor, with simple constitution by controlling the charging current supplied to the capacitor.

A constant current source 121 is connected between the terminal T3 where the capacitor C1 is connected and the collector of a transistor TR31 and the charging current for charging the capacitor C1 is set. The switching of the transistors TR31 is controlled by a control circuit 130. When a switch 131 is turned on, the constant voltage generated by a constant voltage source 132 is applied to A terminal T2 to turn on the transistors TR31. A specific current flows in from the constant current source 121 and the charging current of the capacitor C1 decreases. The time needed for the potential of the capacitor C1 to reach a potential detected by an oscillation circuit 13 for discharging is delayed and then the frequency of the switching control signal outputted from the oscillation circuit 13 for discharging to a terminal 8 decreases.