A rotation amount detecting device for a rotation body includes: a first rotation body having a first surface; a second rotation body having a second surface formed of a material different from that of the first surface and contacting the first surface, and rotating with rotation of the first rotation body; a first support member rotatably supporting the first rotation body at a first position and supporting thereof to be swingable around a first axis at a position different from the first position; a second support member rotatably supporting the second rotation body at a second position and supporting thereof to be swingable around a second axis provided at a position different from the second position, the second axis being the first axis or an axis in the first support member other than the first axis; and a detector that detects a rotation amount of the second rotation body.