A rotation detecting device includes first and second members that are rotatable relative to each other around a rotation axis, a magnetic member fixed to the first member and having poles alternately arranged around the rotation axis, a detecting member arranged to face the magnetic member and made of a magnetic material, and a holding member fixed to the second member. The holding member is made of thermoplastic resin and includes first and second holding members that are welded and fixed to each other. The detecting member is disposed between the first and second holding members to prevent relative contact between the detecting member and the second member. The rotation detecting device further includes a magnetic sensor that detects relative rotation between the first and second members by sensing a change of magnetic field in the detecting member due to the relative rotation.