PURPOSE: To enable a speed detecting device used for the speed controller, etc., of a motor to detect the accurate rotating speed of the motor even when chattering occurs at the changing points of the output signals of sensors.

CONSTITUTION: A speed detecting device is provided with a first sensor 8, second sensor 9 which outputs signals delayed in phase by 90° from the output signal of the first sensor 8, RS latch 12 which is set by the leading edge of the output signal of the first sensor 8 when the output signal of the second sensor 9 is 'L' or by the trailing edge of the output signal of the first sensor 8 when the output signal of the second sensor 9 is 'H', and up counter 13 which measures the generating period of the leading edge of the output signal of the latch 12.