This invention provides a sealed secondary battery comprising a current cutoff mechanism. The current cutoff mechanism has a current breaking valve 30 comprising a sloped side wall 32A which tapers with decreasing diameter from the inner circumference of a flange 32 and a dome 32B descending in a spherical cap shape from the rim of sloped side wall 32A. The rotation axis L of sloped side wall 32A and the sloped side wall 32A form an angle θ to satisfy 60°≤θ≤75° while dome 32B has a sphere radius R of 30 mm or larger, but smaller than 100 mm. In a planer view along the rotation axis L, the outer circumference of dome 32B is located outside the outer circumference of a thin portion 76.