To provide a rotation detecting device with a large maximum axial run-out, superior in corrosion resistance, low dusting characteristic, long service life, reliability, and rotation information detection of a rotary body, capable of detecting not only rotational frequency but rotating direction, rotation angle position, and rotating speed, and capable of using both magnetic and non- magentic materials as a rotor side target material.

This device is provided with a rotation detector 4 detecting a rotation of the rotary body 1 without contact, an axial run-out detector 8 detecting an axial run-out of the rotary body 1, and a signal processing means (a computing unit 11) processing a rotation detection signal from the rotation detector 4 and an axial run-out detection signal from the axial run-out detector 8 for removing an axial run-out component from a signal component of the rotation detection signal.