An embodiment of the present invention relates to a cylindrical lithium ion secondary battery. The technical problem to be solved is to provide a cylindrical lithium ion secondary battery which is designed such that a safety vent is formed and at the same time or afterwards excess metal can be guided to a curved portion adjacent to the safety vent, thereby enhancing the flatness of a can floor and reducing damage to the safety vent. To this end, the present invention provides a cylindrical lithium ion secondary battery comprising: a cylindrical can; an electrode assembly accommodated in the cylindrical can; and a cap assembly for sealing the cylindrical can, wherein the cylindrical can comprises a circular floor portion, a curved portion curved from the floor portion towards the electrode assembly, and a safety vent formed on the curved portion.