TECHNOLOGY FOCUS - ELECTRONICS - Preferred Components: The cap plate includes short side and long side corresponding to the case, and the bending inducing groove is disposed on respective sides of the opening on a same line. The bending inducing groove is disposed in parallel with the short side. The bending inducing groove includes: a vertical surface vertically disposed relative to a top surface of the cap plate and a slanted surface extending from a bottom of the vertical surface to the top surface of the cap plate with a predetermined angle. The bending inducing groove includes first bending inducing groove disposed at a top surface of the cap plate and a second bending inducing groove disposed at a bottom surface of the cap plate corresponding to the first bending inducing groove. First thickness T1 that is set at a lowest bottom of the first bending inducing groove is greater than a second thickness T2 that is set at a highest top of the second bending inducing groove. The first bending inducing groove includes first vertical surface vertically disposed relative to a top surface of the cap plate and a first slanted surface extending from a bottom of the first vertical surface to the top surface of the cap plate with a predetermined first angle, and the second bending inducing groove includes second vertical surface vertically disposed relative to a bottom surface of the cap plate and a second slanted surface corresponding to the first slanted surface and extending from the top of the second vertical surface toward the bottom surface of the cap plate with a predetermined second angle. The first angle is greater than the second angle. The opening corresponds to a vent hole in which a vent plate that is opened when an internal pressure of the case exceeds a predetermined value is installed, and the bending inducing groove is connected to a side of the cap plate on respective sides of the vent hole. The bending inducing groove extends continuously from the opening to a side of the cap plate. The bending inducing groove includes several discrete grooves on each side of the opening, the discrete grooves being disposed along a line from the opening to a side of the cap plate. The opening has vent plate, the vent plate being configured to release an internal pressure of the battery if an internal pressure of the case exceeds a predetermined value, and the opening penetrates the cap plate, and the bending inducing groove includes first and second groove portions that respectively extend from opposite sides of the opening to respective first and second points where opposite edges of the cap plate meet the case, the first and second groove portions each having a depth in the cap plate that is less than a thickness of the cap plate such that the first and second groove portions do not penetrate the cap plate.