PURPOSE: To attain the exact measurement of a frequency by estimating the phase fluctuating quantity of a modulated wave from a signal, which is detected during the measuring period of the frequency, in a receiving side, converting the quantity into a wave number, adding correction to the wave number, which is obtained by counting the rising of the real modulated wave, and obtaining the frequency.

CONSTITUTION: When a demodulated data sequence is received from a demodulator 3, the phase changing quantity during a constant period P is calculated by a phase change estimating circuit 7 based on the structure of the modulated wave. Then, a value (n), for which the phase change quantity (radian) is divided by 2π and converted into the wave number, is outputted. When an obtained wave number N and the converting value (n) are inputted to a frequency estimating circuit 8, in this circuit, the calculation of (N-n)/T is executed and a carrier frequency fc is outputted from a frequency output terminal 9. Thus, since the phase change quantity is estimated from the detected signal and the correction of the measured wave number is executed, the frequency can be exactly measured.