A nonaqueous electrolyte secondary battery comprising: a flat wound electrode body that is formed by making an elongated belt-shaped positive electrode and an elongated belt-shaped negative electrode overlap each other with an elongated belt-shaped separator interposed therebetween to obtain a laminate and winding the laminate in a longitudinal direction of the positive electrode and the negative electrode, in which the flat wound electrode body has a flat portion, a first rounded portion (R 1), and a second rounded portion (R 2), the flat portion has a flat surface and is provided in a center portion in a longitudinal direction of a section of the electrode body perpendicular to a winding axis, and the first rounded portion (R 1) and the second rounded portion (R 2) have a curved surface and are provided in opposite end portions, between which the flat portion is interposed, in the longitudinal direction of the section, a nonaqueous electrolytic solution; and a quadrilateral battery case that accommodates the flat wound electrode body and the nonaqueous electrolytic solution impregnating the electrode body, wherein the second rounded portion (R 2) is provided on a bottom side of the quadrilateral battery case, and opposite end portions of the second rounded portion (R 2) in a winding axial direction are squashed in a thickness direction of the electrode body to a larger extent than opposite end portions of the first rounded portion (R 1) in the winding axial direction so that the distance between portions of the electrodes adjacent to each other in the opposite end portions of the second rounded portion (R 2) is shorter than the distance between portions of the electrodes adjacent to each other in the opposite end portions of the first rounded portion (R 1), characterized in that the opposite end portions protrude towards the bottom of the case closer than a center portion of the second rounded portion (R 2). a flat wound electrode body that is formed by making an elongated belt-shaped positive electrode and an elongated belt-shaped negative electrode overlap each other with an elongated belt-shaped separator interposed therebetween to obtain a laminate and winding the laminate in a longitudinal direction of the positive electrode and the negative electrode, in which the flat wound electrode body has a flat portion, a first rounded portion (R 1), and a second rounded portion (R 2), the flat portion has a flat surface and is provided in a center portion in a longitudinal direction of a section of the electrode body perpendicular to a winding axis, and the first rounded portion (R 1) and the second rounded portion (R 2) have a curved surface and are provided in opposite end portions, between which the flat portion is interposed, in the longitudinal direction of the section, a nonaqueous electrolytic solution; and a quadrilateral battery case that accommodates the flat wound electrode body and the nonaqueous electrolytic solution impregnating the electrode body, wherein the second rounded portion (R 2) is provided on a bottom side of the quadrilateral battery case, and opposite end portions of the second rounded portion (R 2) in a winding axial direction are squashed in a thickness direction of the electrode body to a larger extent than opposite end portions of the first rounded portion (R 1) in the winding axial direction so that the distance between portions of the electrodes adjacent to each other in the opposite end portions of the second rounded portion (R 2) is shorter than the distance between portions of the electrodes adjacent to each other in the opposite end portions of the first rounded portion (R 1), characterized in that the opposite end portions protrude towards the bottom of the case closer than a center portion of the second rounded portion (R 2).