NOVELTY - A square lithium ion secondary battery comprises rectangular parallelepiped battery case with flat cover material which seals opening of rectangular box-shaped case main body and the full circled welding of case main body, positive electrode plate, negative electrode plate, and separator which are winding electrode bodies made into form which is wound by flat shape and contains a flat part and arc part located in both ends.    USE - A square lithium ion secondary battery for power supply of motor vehicle.    ADVANTAGE - Battery is highly safe.    DETAILED DESCRIPTION - A square lithium ion secondary battery comprises rectangular parallelepiped battery case with flat cover material which seals opening of rectangular box-shaped case main body and the full circled welding of case main body, positive electrode plate, negative electrode plate, and separator which are winding electrode bodies made into form which is wound by flat shape and contains a flat part and arc part located in both ends. Winding electrode body is accommodated in battery case in which direction which goes to other arc part from one arc part of electrode body, and direction which goes to the opening from the bottom face of case main body correspond. The dimension of height direction which corresponds the dimension of width direction which corresponds to axial direction of winding electrode body is set to A (mm), direction which goes to opening from bottom face of case main body is set to B (mm), and distance from inner surface of battery case to winding electrode body is set to C (mm). Value in inner area of quadrangle which tied 4 points of (A/B,C) is (1.3,1), (2.2,2.5), (2.2,7), (1.6,7) in a straight line. Product (E1x T) of the Young's modulus (E1) (GPa) of the substance which contains case main body, and thickness (T(mm)) of case main body is greater than or equal to 28. Product of the depth-of-penetration (Dmin(mm)) of case main body and cover material and Young's modulus (E2) (GPa) of substance which contains welding part is greater than or equal to 20.    DESCRIPTION OF DRAWING(S) - The drawing shows a graph of the range with appropriate combination (A/B, C) of the value of A/B, and the value of C. (Drawing includes non-English language text).