PURPOSE: To enable a frequency of a differential PSK signal which is phase- modulated to be measured accurately.

CONSTITUTION: A frequency of differential PSK signal is multiplied by a multiplication circuit 12, only a signal component of the frequency which is multiplied is taken from an output signal of the multiplication circuit 12, and then the output signal which is taken out is frequency-divided by an order of multiplication of a multiplication circuit 12 by a frequency-dividing circuit 12. Then, the frequency-divided signal is transmitted through a signal gate circuit 3 and a counter circuit 7, thus enabling a carrier wave frequency to be calculated at an operation display portion 8. Also, the frequency-dividing circuit 15 is eliminated and the result which is obtained at the operation display portion 8 is divided by the order of multiplication, thus obtaining a proper carrier frequency.