A tray locking mechanism includes a tray on which a sheet is disposed, a moving head that processes the sheet that is disposed on the tray as the moving head moves along the sheet, a tray locker that locks the movement of the tray, and a housing that houses the tray and includes a retainer. The tray locker includes a retaining claw that locks the movement of the tray by locking to the retainer. While the moving head is positioned at a position away from a stand-by position, the retaining claw is locked to the retainer as a result of the moving head separating from the tray locker, and while the moving head is positioned at the stand-by position, the retaining claw is disengaged from the retainer as a result of the moving head pushing the tray locker.