A rotation angle detecting device includes a permanent magnet member, a pair of magnetic sensor elements disposed in a magnetic field formed by the permanent magnet member to provide a pair of output voltage signals when the magnetic field changes as the rotating object rotates; and a rotation angle calculating unit. The rotation angle calculating unit calculates a phase difference between the pair of output voltage signals from a magnitude of the output signal of one of the magnetic sensor elements when detecting a specific magnitude of the output signal of the other magnetic sensor elements whose phase angle is known.