A sheet detecting device includes a rotation unit having an abutment surface, a positioning unit configured to position the rotation unit in a standby position where the leading edge of the conveyed sheet abuts the abutment surface, and a detecting unit configured to detect the conveyed sheet on the basis of the rotation of the rotation unit pressed by the conveyed sheet, wherein the rotation unit rotates to a sheet passage posture where the sheet is allowed to pass after being pressed by the leading edge of the conveyed sheet and, when the trailing edge of the conveyed sheet passes the rotation unit, the rotation unit is rotated from the sheet passage posture in the same direction as a sheet conveying direction and is positioned in the standby position.