PURPOSE: To obtain optimum responsibility by acquiring abrupt variation in the frequency of input pulses accurately when the number of division pulses at gate time increases and the number of registers of a storage part increases.

CONSTITUTION: Pulses inputted from an input terminal 15 are counted by a counter 17, whose output is inputted to a register 181 of the storage part successively. Then the comparator of a decision part 21 compares the number of pulses (contents of registers 181∼184) of the 1st decision block including the up- to-date number of pulses (contents of register 181) with the number of pulses (contents of registers 185∼188) of the 2nd decision block which is counted 5/n∼ 8/n periods before at intervals of 1/n gate time, and when the difference in the number of pulses obtained by the decision part 21 is larger than, for example, "2", the decision part 21 makes a decision on large variation to rewrite the contents of the registers of the storage part 18. Therefore, when variation width of an input frequency (number of pulses) is "20", a decision on large variation is made to perform rewriting, thus improving the responsibility.