The purpose of the present invention is to provide a safety mechanism that releases pressure inside a battery when battery errors occur, in a small, cylindrical lithium ion secondary battery. This cylindrical lithium ion secondary battery has, housed inside a battery case, an electrolyte and an electrode group comprising a separator wound or laminated between a positive electrode and a negative electrode and is sealed by a sealing body. The cylindrical lithium ion secondary battery is characterized by: the external diameter of the battery case being no more than 10 mm; the sealing body having a ring having a through-hole and a sheet- or film-shaped valve body arranged so as to cover the ring through-hole; comprising a safety mechanism whereby the valve body cleaves when the battery interior reaches a prescribed pressure; the ring having a plurality of protrusions that protrude towards the inside of the through-hole; and the protrusions comprising a linear ridgeline.