A rotation detecting device includes a plurality of magnetic encoders disposed coaxially and having respective numbers of magnetic poles different from each other and a plurality of magnetic sensors for detecting respective magnetic fields emanating from those magnetic encoders. Each of the magnetic sensors is in a ring form and obtains the position information within the magnetic poles of the associated magnetic encoder. A phase difference detector determines the difference in phase between magnetic field signals detected respectively by the magnetic sensors. An angle calculator is provided, which is operable on the basis of the difference in phase so detected to calculate an absolute angle of each of the magnetic encoders. A corrector corrects the initial phase difference occurring in the magnetic field signals, detected respectively by the magnetic sensors, as a result of the fitting position of the magnetic encoders.