ALKALINE SECONDARY BATTERY

A nickel-metal hydride secondary battery 2 is provided with: an outer package can 10 having an opening at an upper end; an electrode group 22 accommodated in the outer package can 10 together with an alkaline electrolyte; and a sealing body 11 that seals the opening of the outer package can 10. The sealing body 11 includes a cover plate 14 having a central through hole 16, and a valve element 18 made of an elastic material to close the central through hole 16. The valve element 18 includes a cylindrical main body part 54 having a base end surface 58 that covers the central through hole 16, and an extended diameter part 56 positioned on an opposite side of the main body part 54 from the base end surface 58 and having a diameter made larger than a diameter of the main body part 54. A ratio R of a thickness T of the extended diameter part 56 to a total height H of the valve element 18 is equal to or higher than 27%.