To obtain a servo valve having high reliability using a feedback mechanism having no effect of electromagnetic noise, by making a laser beam be incident onto a light reflecting part provided on a movable part connected to a spool, and also arranging a laser beam leading part for receiving reflected light opposite to the reflecting part.

A position detecting part is composed of a light reflecting mirror 14 fitted to one side end surface of a spool 12, and a light leading part 15 for irradiating a laser beam to the mirror 14 and fitted to the case of a servo valve for receiving this reflected light. The following are composed a laser interferometer 27 having one component of the mirror 14 fitted to the spool 12 of the position detecting part, and a signal treating part 28 for adjusting and counting an output waveform from the interferometer to manipulate the output waveform into a displacement output signal. The reflected light received in a laser beam leading part is led to the interferometer 27, and the fine displacement quantity of the spool is measured by the interferometer 27 to be transmitted to an information treating part as a feedback signal by an optical fiber.