PURPOSE: To adjust the oscillating frequency by selecting a binary code of selection terminals so as to turn on a proper MOS switch thereby selecting taps of oscillation resistors.

CONSTITUTION: Four states are selected for selection terminals 19, 20 of a decoder circuit 21 by using a binary code and in turning on MOS switches 11∼14 one by one, the tap position of a built-in resistor 10 is selected. Since the resistance is varied therewith, the oscillation frequency is changed from the oscillation frequency fosc ∝ 1/(CR) to adjust the frequency. In an actual IC, since it is manufactured by a value close to the designed resistance, the tap position is provided in the vicinity of the resistance and slight adjustment is applied, for example, eight kinds of selections are attained by three terminals and 16 kinds of selections are attained by four terminals, and minute adjustment is applied. Conversely, a wide tap position is adopted and the oscillating frequency is changed largely in the same CR oscillator to output various frequencies as another application.