The present invention relates to a secondary battery including an insulating case interposed between the cap assembly and the electrode assembly. The insulating case is made of a sealing material with low thermal deformation, excellent flame-retardation, and excellent electric insulation so as to be able to enhance the reliability of the secondary battery. The secondary battery of the present invention includes a can having an open top, an electrode assembly disposed inside the can and generating electricity, a cap assembly sealing the open top of the can, and an insulating case interposed between the cap assembly and the electrode assembly. The insulating case includes a base having an upper surface and a side surface, and a protrusion formed on the side surface. The upper surface has a deformation hole. The deformation hole and the protrusion are arranged in a manner that a line connecting the center of the deformation hole to the protrusion is substantially perpendicular to the side surface.