Abstract: An oscillator circuit incorporated in a semiconductor integrated circuit and comprising an oscillation input terminal, an oscillator output terminal, an inverting logic circuit, and a transmission gate. The terminals and are connected to an external oscillator. The inverting logic circuit comprises an N-type MOS transistor and a P-type MOS transistor. The input and output terminals of the logic circuit are connected to the oscilllation input and output terminals, respectively. The transmission gate comprises an N-type MOS transistor and a P-type MOS transistor. Either MOS transistors has a source electrode and a drain electrode, one of which is connected to said oscillation input terminal, and the other of which is said oscillation output terminal. The gate-insulating film at least one of said MOS transistors incorporatedin the transmission gate is thicker than those of other MOS transistors formed on the semiconductor substrate.