A secondary battery may include a cell body member accommodating an electrode assembly therein, including three sealing sides and provided adjacently to a cooling plate member; and wherein the cell body member, in contact with a heat conductive member on a lower surface thereof comprises a surface area-increasing groove formed to be concave in the lower surface thereof and the lower surface is a region in which a sealing portion is not formed, wherein the heat conductive member provided in at least a portion between the cell body member and the cooling plate member to form a heat path for transferring heat from the cell body member to the cooling plate member, wherein the surface area-increasing groove includes a curved-region in a cross section in a thickness direction of the cell body member, wherein the curved-region is in contact with the heat conductive member.