PURPOSE: To surely achieve the calking even when a fitting flange is tilted by providing a calking arm, a holding tool to temporarily hold a corner piece to a head part, and a pressing arm to press the corner piece against each fitting flange prior to the machining action of the calking arm to connect the corner piece at the correct position.

CONSTITUTION: A corner piece C which is temporarily held using a temporary holding tool 30 is correctly fitted to the inner side of each fitting flange F and a lip piece L by pressing a plate part and an inflated rib C3 close to an inner edge C1 using inner edge abutting parts 41a, 51a, and rib abutting parts 41b, 51b of pressing arms 10, 50 to be operated prior to the calking action of working frames 2, 3. When the fitting flange F is tilted, the clearance between the opposite surfaces of an outer edge C2 part of the corner piece C and the fitting flange F is eliminated to surely achieve the calking by the force to erect the fitting flange F at a head part to the right angle and the pressing force at the rib abutting parts 41b, 51b.