To reduce the variation of oscillation frequency by connecting a 2nd capacitor between even inverters and a Schmitt inverter and increasing the amplitude of an obtained oscillation output, based on a prescribed time constant.

When a charging/discharging waveform A reaches threshold voltage Vth, the output B of a Schmitt inverter 6 falls to a low level and the output C of an inverter 7 is turned to a high level. Since a capacitor 11 capacity-couples the output C and the input of the inverter 6, a waveform A appearing on the non-ground side is synchronized with the output C and tries to rise up to power supply voltage Vdd. Since the voltage boosting width of the RC oscillation circuit is clipped by a diode 12, the destruction of the circuit is prevented. When the output of an inverter 8 is turned to a low level, the capacitor 10 starts discharge by a time constant determined by the capacitance of the capacitor 10 and the resistance value of a resistor 9. Thereby, the output C is turned to the low level and the waveform A tries to fall only by the voltage Vdd. The falling width is clipped by a diode 13.