To prevent generation of a sliding flaw of the inner face of an opening end part by allowing the outer peripheral part of a guide block at the position in contact with the opening part to follow the opening end corresponding to the abutting force of the opening part.

A guide block 11 is stopped while being inserted into an opening end of a can C. The opening end is squeezed by a necking die 9. During the opening end enters into contact with the outer peripheral part of the guide block 11 and the opening part enters between the guide block 11 and the necking die 9, at least the outer peripheral part of the guide block 11 at the position in contact with the opening end is made to follow the opening end corresponding to the abutting force of the opening end. The guide block 11 is inserted into the opening part of the can by actuating a spindle 8 and energized toward the tip direction by an elastic body 14. By this method, speed up of a can production line and thinning of a can are thorouly attained.