PURPOSE: To determine an average value by a simple circuit with one counter, by a method wherein a pulse train of the first period is delivered in the term from a reference signal to the first signal, a periodic pulse doubling that in the first term is delivered in the term from the first signal to the second, and the pulses are counted.

CONSTITUTION: A repeating pulse of a period P for measuring terms T01 and T02 are given to one input of a data selector 3, while a pulse of a period 2P is given to the other input of the data selector 3 via a 1/2 frequency divider 2. The data selector 3 delivers to an AND gate a periodic pulse of 2P in the term T02- T01=t2-t1=T12 and a periodic pulse of P in the other term according to a signal (c) of a comparator 6 obtained by shaping a signal waveform (b) based on a control input, and the gate is opened by a pulse of the term T02 from FF7 for counting 5. Accordingly, an average value of t1 and t2 is determined by a simple circuit wherein one counter 5 is used.