PURPOSE: To adjust the frequency of an osillator independently of measurement without providing a special measuring terminal to an output terminal by outputting selectively an output of the oscillator on a semiconductor substrate to the output terminal for frequency measurement via a buffer.

CONSTITUTION: The oscillator is provided on the semiconductor substrate and an output of the oscillator is inputted to a selecting circuit 10 via the buffer 9. Further, a control signal 12 is inputted to the selecting circuit 10, where an output of the oscillator and other signal output 11 are selected by the signal 12 and the result of selection is obtained at an output terminal 13. Moreover, an integrated circuit having the oscillator is provided with a crystal oscillator 2, a feedback resistor 5, and capacitors 3, 4 or the like and the frequency of the oscillator is adjusted by varying the capacitor. Then, the oscillated frequency of the oscillator outputted at the output terminal 13 is measured without being affected by the frequency adjustment. Further, no special terminal for the frequency measurement is provided, allowing to simplify the constitution of the integrated circuit.