A sheet aligning device includes a support member, a regulation member, a regulation member moving mechanism, and an alignment member. The support member supports leading ends of sheets. The regulation member is connected to the support member and is provided to face an inclined tray in such a way as to define a sheet entry space in which the leading ends of the sheets enter. The regulation member moving mechanism is configured to, each time a sheet is conveyed onto the inclined tray, move the regulation member in a separating direction to widen the sheet entry space, and then in an approaching direction to narrow the sheet entry space. The alignment member aligns the sheets by abutting on rear ends of the sheets.