A speed detecting device comprises an optical pattern (7a) formed on a moving member; pairs of a light emitting means (5) and a light receiving means (6) disposed on a stationary member; a signal means (4) which generates periodic wave signals from the outputs of the light receiving means (6), the periodic wave signals being different in phase; a differential means (9a-9d) which differentiates the periodic wave signals to produce differentiated signals different in phase; a comparing means (10a,10b) which produces comparison signals from combinations of the periodic wave signals, the comparison signals being different in phase; a peak signal means (11a,11b,12a-12d) which produces peak signals from the comparison signals corresponding respectively to the peak regions of the differentiated signals; and a selection means (13a-13d) which selectively outputs the peak regions of the differentiated signals in accordance with the peak signals.