A sheet width aligning device includes two width aligning members which respectively contact widthwise ends of a sheet to regulate a widthwise position of the sheet. A first detector detects one widthwise end of the sheet and a second detector detects the other widthwise end of the sheet. A moving mechanism moves the first and second detectors located at initial positions at outer sides of the sheet towards the center of the sheet in the widthwise directions. A determining section determines the positions of the width aligning members based on a detection timing, at which the first detector detected the one widthwise end, and a detection timing, at which the second detector detected the other widthwise end. A driving mechanism drives the width aligning members to move to the positions determined by the determining section.