A sheet stacking device includes a tray having a sheet loading surface on which a sheet fed in a predetermined feeding direction is loaded. The tray includes a main tray, an extension tray and a first detection part. The first detection part includes a first sensor, a first detected piece and a first contact piece. The first detected piece is turnably supported by the extension tray and switches the first sensor between an ON state and an OFF state. The first contact piece is turnably supported by the first detected piece, and protrudes above and retracts below the sheet loading surface.