A rotation measuring mechanism capable of measuring a rotation amount and a displacement amount of a rotary member in a direction perpendicular to the rotation axis is provided. An exemplary combination of the rotation measuring mechanism includes a detection target rotating in synchrony with the rotary member and range sensors. Each time the detection target rotates once, each sensor outputs one cycle of a sine wave. The amplitude of the sine wave output from the sensor varies in accordance with the displacement of the detection target together with the rotary member in the direction perpendicular to the axis, thereby obtaining the rotation amount and the displacement amount of the rotary member.