A paper sheet folding device (sheet folding device) 160 includes a paper sheet folding unit 60 and a tray unit 110 that is provided on one side surface 161a of a device main body 161 and includes a lower discharge tray 121 on which a paper sheet S discharged through a lower discharge port 85 is loaded. The tray unit 110 is pivotable in an up-down direction about a pivotal shaft 111. A discharge drive mechanism 132 is disposed at a center portion of the tray unit 110 in a paper sheet width direction.