A cap assembly of a secondary battery is provided. The cap assembly includes: a cap plate; a current interrupt device (CID) under the cap plate, the CID including: a vent plate under the cap plate, the vent plate including a vent protruding downward and to be deformed when internal pressure of the secondary battery is increased; and a sub-plate under the vent plate and connected to the vent; a middle plate between the vent plate and the sub-plate and electrically connected to the vent plate through the sub-plate, the middle plate having a receiving groove to receive the sub-plate; and an insulator between the vent plate and the middle plate.