PURPOSE: To provide a calculation method for pulse signal cycle which can reduce the absolute value of errors caused by the calculation of the pulse signal cycle in comparison with the conventional method.

CONSTITUTION: The number of edges is calculated based on the edge interruptions and a pulse signal cycle 22 is calculated based on the clock pulse which oscillates a reference frequency. In such a calculation method, the multiple of the reciprocal of a prescribed number of the clock pulse cycle is added to the integrated value of clock pulse cycles set among the edges for correction of this integrated value. Thus the cycle 22 can be calculated.