PURPOSE: To prevent an interruption processing for frequency measurement from being performed unnecessarily by inhibiting the interrupting operation when the 2nd signal is set so that a signal to be measured is applied to a timer means in response to the 2nd signal.

CONSTITUTION: The 1st means 5 which outputs the 1st signal F1 is set corresponding to the specific timing of the signal S to be measured and reset when calculation is performed by a calculating means 3. Further, the 2nd means 6 which outputs the 2nd signal F2 is set in response to the specific timing of the signal when the signal F1 is set and reset when the calculation is carried out by the means 3. When the signal F2 is set, the interrupting operation is inhibited by a mask means 7 so that the signal S is not applied to a timer means 1 corresponding to the signal F2. Therefore, the period is measured not as many times as at least the calculation frequency of the means 3, so the interrupting operation for the frequency measurement is minimized.