A medium placing device (feeding tray) includes a placement unit, a regulation unit (edge guide), a displacement unit, a detection unit, and an adjustment unit. The regulation unit (edge guide) includes a first regulation member (first edge guide) and a second regulation member (second edge guide). The displacement unit is capable of moving the first regulation member (first edge guide) and the second regulation member (second edge guide) in directions opposite to each other along a first direction. The detection unit is provided between the placement unit and the adjustment unit.