A sheet positioning device includes a sheet setting plate, first and second regulating members disposed facing each other to move in an orthogonal direction to a sheet conveyance direction to regulate positions of two facing ends of the sheet on the plate by contacting these ends, a driving mechanism to move the first regulating member in the orthogonal direction, first and second contact detectors mounted on the first and second regulating members to detect contact of the two facing ends, and a controller to stop the driving mechanism based on detection results obtained by the first and second contact detectors. The first and second regulating members are used to adjust a position of the sheet to a predetermined position. Respective contact surfaces of the first and second contact detectors with the sheet extend over an entire maximum loadable range In a sheet setting direction on the plate.