PURPOSE: To simplify the circuit and to stabilize the oscillation by providing a shift circuit shifting a terminal voltage of a capacitor for charge/discharge to a high and a low level.

CONSTITUTION: When a switch 51 is turned on and a switch 52 is turned off, the terminal voltage across a capacitor 56 rises gradually. When the voltage exceeds a reference value, the terminal voltage rises rapidly by a shift circuit 54, and then the switches 51, 52 are changed over. Thus, the discharge of the capacitor 56 is started via the switch 52 and the terminal voltage descends gradually. When the value goes down lower than a reference value, the terminal voltage falls down rapidly by the shift circuit 55, the switches 51, 52 are changed over again and the capacitor 56 is charged again. The operation above is repeated. Since one reference value is enough, the circuit is simplified and the oscillation is stabilized.