A method for producing a stable control signal for impedance matching is provided which is capable of suppressing variation in impedance matching data by adding a shift voltage to a voltage to be compared. A comparator compares the voltage to be compared with a reference voltage and an up-down counter performs a counting operation according to a result from the comparison. A code converting circuit converts a count value output from the up-down counter to a thermometer code used for changing an impedance of an impedance varying circuit. A change in the impedance is made in a manner that, even when the voltage to be compared gets closest to the reference voltage, a shift voltage for the comparator to make an exact comparison is fed to the voltage to be compared. An averaging circuit averages a count value and the code converting circuit converts a resulting average value to the thermometer code.