PURPOSE: To perform an easy and precise position detection by providing first and second marker detectors, a narrow pulse signal generating means, a counter signal generating means and an UP/DOWN counter.

CONSTITUTION: In a moving quantity detecting device 10, AS, AE, BS, BE signals which are pulse signals sufficiently narrower in width than the moving time for one area of a rotating part are generated every area of the rotating part which is the interval between a first marker detector 1 and a second marker detector 2 by a narrow pulse generating means 3, an UP signal and a DOW signal are generated by a count signal generating means 4 from CHA signal and CHB signal whose phases are mutually switched according to the rotating direction of the rotating part and the AS, AE, BS, BE signals, and increment/decrement of the UP signal/DOWN signal is performed by an UP/ DOWN counter 5. Therefore, the precise position of a moving body can be detected by the detecting device 10 with a simple structure.