PURPOSE: To eliminate virtually the dependence on the powder supply voltage for an oscillation frequency by using a constant voltage generating circuit which generates the constant voltage to an inverter from a power supply and then impresses the constant voltage to the gate of an MOS transistor for supply of constant current.

CONSTITUTION: An MOS transistor (TR)21 for supplying the power to a CMOS inverter 6 and a constant voltage generating circuit 23 which generates the constant voltage to the inverter 6 from a power supply and impresses this constant voltage to the gate of the TR21 are provided. In this case, an end of the TR1 side of a series circuit of TR1 and TR2 is connected to a power supply 3 via a p type constant current supply TR21. Thus the TR21 is set into the current supply path of the inverter 6. In this case, a drain current Id flowing to the TR21 is set approximately at a constant level. As a result, the voltage Vx which is impressed to the inverter 6 does not virtually depend on the power supply voltage Vd by using Ra/(Ra+Rb) obtained from resistance values Ra and Rb of resistances 26 and 27 of the circuit 23 as a variable constant.