Abstract: A plastic liquefying device, includes a gasifying furnace (19) for generating pyrolytic gas by heating and melting plastic waste and a liquefying tank (2) for liquefying and separating the pyrolytic gas generated in the gasifying furnace (19). The gasifying furnace (19) further comprising a heating device (22) for heating the inside of an oven-shaped furnace body (20) provided at the bottom of the furnace body (20) having a plastic waste inlet (H) provided at the top thereof and an opening and closing mechanism (21) for opening and closing the inlet (H) installed at the inlet (H). The opening and closing mechanism (21) further includes a plate shutter (24) larger than at least the opening area of the inlet (H) and a fixing device (25) for pressingly fixing the shutter (24) to the seal surface (S) around the peripheral edge part of the inlet (H).