A secondary battery including: an electrode assembly including a first electrode plate, a second electrode plate, and a separator interposed between the first electrode plate and the second electrode plate; a can accommodating the electrode assembly and an open end; and a cap assembly to seal the open end of the can. A step difference is formed in an upper end of the can. A supporter settled in the step difference is formed in the cap assembly. A bottom protrusion inserted into the can is formed on a bottom surface of the supporter. Therefore, ignition and explosion are prevented by a short between the electrode plates in accordance with the deformation of the electrode assembly so that the stability of the secondary battery may be improved.