PURPOSE: To obtain a system which reproduces data in an easy method without using any filter, by finding the differential value of a waveform from the difference between two succeeding samples, and then comparing the sum of squares of said differential value and a sampled value with a reference value.

CONSTITUTION: An FSK signal is delayed successively through delay elements 1 and 2 to obtain sampled values xn-1, xn, and xn+1, and a difference circuit 3 obtains the difference between the current value, and the sampled values before and after it. A multipler 4 multiplies the difference by a factor 1/2 and thus weight it to obtain a cosine value yn. This cosine value yn and the current value xn of a sine wave are inputted to squaring circuits 5 and 6, whose output results are added together by an adder 7. This addition value An2 varies like a positive or negative pulse at a frequency variation point of the FSK signal according to the directions of frequency variation, so the output of the adder 7 is compared with a positive threshold value VH and a negative threshold value to control a flip-flop 10 on the basis of the comparison outputs, obtaining reproduced data.