PURPOSE: To devise the circuit so as to change over even a minute capacitance less than a stray capacitance of a switch and a wire by adopting the constitution such that plural series circuits each comprising a capacitor and a switch and at least one of series circuits each comprising a capacitor, a resistor and a switch are connected in parallel.

CONSTITUTION: Plural series circuits each comprising a capacitor 125-127 and a switch 115-117 and at least one of series circuits each comprising a capacitor 121-124, a resistor 131-134 and a switch 115-117 are connected in parallel. Since the resistor in the series connection of the capacitor and the resistor limits the charge/discharge speed of the capacitor, the effective capacitance is changed depending on the frequency. The relation between the capacitance of the entire circuit and the frequency depends on the capacitor at a low frequency but the capacitance is reduced effectively through less carrier charged/discharged from/to the capacitor because the succeeding discharge period is transited before the charging of the capacitor is finished at a high frequency. Thus, even a minute capacitance is changed over without the effect of a stray capacitance by selecting a capacitor and a resistor in matching with the operating frequency.