The invention provides an external corner cover A includes at least two plate pieces 1a and 1a bonded with each other at side portions thereof via reactive hot melt adhesive P so as to form an apex angle portion 3 extending in a longitudinal direction. A chamfered portion 8 with a narrow width is formed on the apex angle portion 3. The chamfered portion 8 is formed into a longitudinally extended non-horizontal irregularly arranged continuous concavo-convex surface. An adhesive layer P remains at a joint portion of the apex angle portion 3 and an end face of the adhesive layer P at the chamfered portion side is a flat surface along the concavo-convex surface of the chamfered portion 8.