A sheet thickness detecting device for detecting a thickness of a sheet includes a fixed roller rotatably supported by a shaft so that the fixed roller faces the sheet; a movable roller rotatably provided so that the movable roller and the fixed roller sandwich and feed the sheet, and configured to be displaced in accordance with the thickness of the sandwiched sheet; a swing member rotatably supporting the movable roller and including a shaft so that the swing member is swung around the shaft, wherein the swing member further includes a detection target part formed in a manner that a distance between the detection target part and the shaft is greater than that between a position where the movable roller is supported by the swing member and the shaft; and a detecting unit detecting the displacement amount of the detection target part of the swing member.