A secondary battery includes an electrode group 5 formed by winding a positive electrode plate 1 and a negative electrode plate 2 with a separator 3 interposed therebetween. Each of the positive and negative electrode plates 1 and 2 includes a current collector 1 and mixture layers 8 and 9 each containing an active material and formed on the surface of the current collector 1. A plurality of trenches 30 are formed in the surfaces of the mixture layers 8 and 9 of at least one of the positive electrode plate 1 and the negative electrode plate 2. The trenches 30 have curvature portions at side edges 30b and bottom centers 30a thereof.