A sealed battery cell including an anti-explosion mechanism disposed between an electrode group and a top cover, and that deforms or cleaves due to elevation of internal pressure in the battery cell, the electrode group being comprised of a positive electrode and a negative electrode that are laminated together; wherein the anti-explosion mechanism includes a diaphragm, a connection plate welded to the diaphragm, and a connection lead welded to the connection plate; and the connection plate includes a thinner portion where a spot at which the connection lead is connected is formed as thin, and a thicker portion where the thinner portion is not formed, and a lower surface of the thinner portion is formed as coincident with a lower surface of the thicker portion, or as hollowed out from the lower surface of the thicker portion.