PURPOSE: To manufacture mouths of the can in the roof plate with a high production efficiency, by forming a opened verical section in the roof plate reinforced with a disk placed on the roof plate, and by deforming the vertical section outward, downward, and then, inward.

CONSTITUTION: A circular depressed and stepped section is formed in a roof plate 3 made of a tinplate, etc., and the depressed section is used as a locating section 3a. A reinforcing disk 4 is fitted into the depressed section. After openings 3b and 4a are formed at the central part of the locating section 3a and the reinforcing disk 4 simultaneously, vertical sections 3c and 4b are formed to the locating section 3a and the reinforcing disk 4. Then, these vertical sections 3c and 4b are simultaneously deformed by bending them outward, downward, and then, inward, and arced sections 3c' and 4b' are formed. Since this method performs the installation of openings in the reinforcing disk 4 and the roof plate 3 and the forming at the same time, mouths of the can are manufactured with a high production efficiency.