A system for detecting the rotational speed of a rotating body comprising the steps of generating two kinds of signals A and B having alternating wave forms and corresponding to the rotational speed of the rotating body by means of position-detecting devices (5a, 5b), the signals A and B having the same amplitude and having phases deviating by 90.degree., producing switching signals S(A), S(A), S(B), and S(B) from the signals A and B by means of zero-cross circuits (711, 712), producing inverted signals A and B from the signals A and B by means of inverter circuits (72, 73), producing differentiation signals dA, dA, dB, and dB by supplying the signals A, A, B and B to differentiation circuits (74, 75, 76, 77), and obtaining output signals corresponding to the rotational speed of the rotating body by selectively switching the differentiation signals dA, dA, dB, and dB in a switching circuit 78 by means of the switching signals S(A), S(A), S(B), and S(B), thereby detecting the rotational speed of the rotating body.