An electrode group formed by winding a positive electrode plate and a negative electrode plate with a separator interposed therebetween is housed in a battery case, and a sealing plate seals an opening of the battery case. The sealing plate includes an upper metal plate, a valve, and a lower metal plate that are stacked, an insulating plate is placed on a portion of the electrode group near the opening of the battery case, one of the positive electrode plate or the negative electrode plate of the electrode group is connected to the lower metal plate through a lead, the lower metal plate and the insulating plate have a first opening and a second opening, respectively, and a ratio of S2 is within the range of 1.8-3.3, where S1 represents an area of the first opening, and S2 represents an area of the second opening.