A rotation angle measuring device provided with a fixed unit and a movable unit relatively rotating with respect to the fixed unit, comprising a light source installed on either one of the fixed unit or the movable unit and for emitting a detection light and a reference position signal light, a polarizing plate for converting a detection light emitted from the light source to a polarized light, a polarized light rotating unit for rotating the polarized light around an optical axis of the light source as the center, a reference position signal light emitted at a reference rotating position of the polarized light, a stationary polarizing plate provided on either one of the fixed unit or the movable unit and to stand still with respect to a rotation of the polarized light, a photodetection sensor provided on the fixed unit or on the movable unit and for receiving the polarized light passing through the stationary polarizing plate and the reference position signal light, and an arithmetic unit for calculating a detection waveform of a change of light amount based on a signal from the photodetection sensor, for detecting the reference position signal light and for calculating a relative rotation angle between the fixed unit and the movable unit from a phase of the detection waveform and a predetermined detection reference phase when the reference position signal light is detected.