The first sliding molds are movable back and forth in a direction orthogonal to a back-and-forth direction of the movable mold, come into contact with a periphery of the central mold when the first sliding molds move forward to form a first cavity for molding a panel body, and avoid interference with a second sliding mold when the first sliding molds move back to form a second cavity for molding a frame. The second sliding mold is movable back and forth in the back-and-forth direction of the movable mold, allows a forward movement of the first sliding molds when the second sliding mold moves back to form the first cavity, and comes into contact with an inner peripheral edge and an outer peripheral edge of the panel body held on a fixed mold, thereby forming sealing portions, when second sliding mold moves forward to form the second cavity.