Operations as a variable resistor are favorably realized even when a drain-source voltage of a variable MOS resistor and that of a reference MOS resistor are not the same. A gate voltage of the variable MOS resistor is controlled with reference to a gate voltage which is controlled such that a voltage generated in the reference MOS resistor is controlled to be the same as a reference voltage. A resistor is connected in parallel with the reference MOS resistor between the drain and source thereof, the resistor including a pair of resistors having the same resistance connected in series. Half of a drain-source voltage of the reference MOS resistor is detected at an intermediate point of the pair of resistors. The gate voltage of the variable resistor is obtained by subtracting one-half of the drain-source voltage from the gate voltage of the reference MOS resistor.