A secondary battery comprising: a can configured to accommodate an electrode assembly; and a cap assembly bonded to an opening of the can, wherein the cap assembly comprises: an upper cap provided in the opening and connected to an electrode tab of the electrode assembly; and a lower cap configured to bond the upper cap to the opening, wherein the lower cap comprises: a first bonding part in close contact with the upper cap and integrally bonded to a bottom surface of the upper cap to support the upper cap; and a second bonding part formed on an outer circumferential surface of the first bonding part wherein the second bonding part is in close contact with the upper cap to fix the upper cap while being integrally bonded to the upper cap, wherein the second bonding part extends in a direction of the upper cap to surround an outer circumferential surface of the upper cap, wherein an outer circumferential surface of the second bonding part is in close contact with the opening and is melted by heat to be bonded to the opening. a can configured to accommodate an electrode assembly; and a cap assembly bonded to an opening of the can, wherein the cap assembly comprises: an upper cap provided in the opening and connected to an electrode tab of the electrode assembly; and a lower cap configured to bond the upper cap to the opening, wherein the lower cap comprises: a first bonding part in close contact with the upper cap and integrally bonded to a bottom surface of the upper cap to support the upper cap; and a second bonding part formed on an outer circumferential surface of the first bonding part wherein the second bonding part is in close contact with the upper cap to fix the upper cap while being integrally bonded to the upper cap, wherein the second bonding part extends in a direction of the upper cap to surround an outer circumferential surface of the upper cap, wherein an outer circumferential surface of the second bonding part is in close contact with the opening and is melted by heat to be bonded to the opening. an upper cap provided in the opening and connected to an electrode tab of the electrode assembly; and a lower cap configured to bond the upper cap to the opening, wherein the lower cap comprises: a first bonding part in close contact with the upper cap and integrally bonded to a bottom surface of the upper cap to support the upper cap; and a second bonding part formed on an outer circumferential surface of the first bonding part wherein the second bonding part is in close contact with the upper cap to fix the upper cap while being integrally bonded to the upper cap, wherein the second bonding part extends in a direction of the upper cap to surround an outer circumferential surface of the upper cap, wherein an outer circumferential surface of the second bonding part is in close contact with the opening and is melted by heat to be bonded to the opening. a first bonding part in close contact with the upper cap and integrally bonded to a bottom surface of the upper cap to support the upper cap; and a second bonding part formed on an outer circumferential surface of the first bonding part wherein the second bonding part is in close contact with the upper cap to fix the upper cap while being integrally bonded to the upper cap, wherein the second bonding part extends in a direction of the upper cap to surround an outer circumferential surface of the upper cap, wherein an outer circumferential surface of the second bonding part is in close contact with the opening and is melted by heat to be bonded to the opening.