The invention provides a tray replacing device which does not need to use a forklift to obtain a tray to be replaced or to position the tray in a specified position during tray replacement when replacement should be carried out. A tray replacing device (200) carries a tray (A) carrying cargo into an operation part (P1), guides a tray (B) supplied by a tray supply mechanism (8) into the operation part (P1) through a first working table (40) to replace the tray. The tray supply mechanism (8) has a tray holding mechanism (80) holding the tray (B) in a tray holding space above a standby position of the first working table (40) in a multilayer manner and enabling a tray (B1) at the lowest layer to be separated; a guide rail (16) arranged between the tray holding space (S40) and the first working table (40); a tray supporting body (82) which supports the first tray (B1) of the lowest layer from the lower side and whose base end part (822) is slidably freely supported on the guide rail (16);and a lifting mechanism (83) which enables the tray supporting body (82) to perform lifting along the guide rail (16).