A secondary battery includes an electrode assembly having a first electrode plate, a second electrode plate, and a separator located between the first electrode plate and the second electrode plate; a first collecting plate and a second collecting plate that are electrically connected to the first electrode plate and the second electrode plate, respectively; a case configured to accommodate the electrode assembly, the first collecting plate, and the second collecting plate; and a cap plate configured to close the case to seal the electrode assembly, the first collecting plate and the second collecting plate within the case, wherein reinforcement stepped parts are formed on the cap plate such that a center region of the cap plate has a thickness different from a thickness of peripheral regions of the cap plate.