Let k be an algebraically closed field of characteristic 0. Let X be a 2 × e matrix of linear forms over a polynomial ring k[x 1 , . . . , x n ] (where e, n ≥ 1). We prove that the determinantal ring R = k[x 1 , . . . , x n ]/I 2 (X) is Koszul if and only if in any Kronecker-Weierstrass normal form of X, the largest length of a nilpotent block is at most twice the smallest length of a scroll block. As an application, we classify rational normal scrolls whose all section rings by natural coordinates are Koszul. This result settles a conjecture due to Conca.