A non-aqueous electrolyte secondary battery, wherein the non-aqueous electrolyte secondary battery is a flat stack type laminate battery in which the ratio value of battery area (projected area of the battery including the battery outer casing body) to rated capacity is 5 cm 2 /Ah or more and the rated capacity is 3 Ah or more; and wherein the battery comprises a power generating element including a positive electrode, the positive electrode comprising: a positive electrode current collector; and a positive electrode active substance layer that is formed on a surface of the positive electrode current collector and has a positive electrode active substance containing a composite oxide comprising lithium and nickel and a spinel type lithium manganese composite oxide, wherein, when the average secondary particle diameter of the composite oxide comprising lithium and nickel as measured by laser diffraction is D50 [μm], the content ratio of the composite oxide comprising lithium and nickel in the positive electrode active substance layer is A [% by mass], the average secondary particle diameter of the spinel type lithium manganese composite oxide measured by laser diffraction is D50 [μm], and the content ratio of the spinel type lithium manganese composite oxide in the positive electrode active substance layer is B [% by mass], the positive electrode satisfies the following Mathematical Formula 1 and Mathematical Formula 2: 0.67 ≤ D 50 A / D 50 B ≤ 1.5 0.3 ≤ B / A + B ≤ 0.5; wherein the composite oxide comprising lithium and nickel has a composition represented by General Formula: Li a Ni b Mn c Co d M x O 2, with the proviso that, in the formula, a, b, c, d, and x satisfy 0.9 ≤ a ≤ 1.2, 0.44 ≤ b ≤ 0.51, 0.27 ≤ c ≤ 0.31, 0.19 ≤ d ≤ 0.26, 0 ≤ x ≤ 0.3, and b + c + d = 1, M represents at least one selected from a group consisting of Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, and Cr. wherein the non-aqueous electrolyte secondary battery is a flat stack type laminate battery in which the ratio value of battery area (projected area of the battery including the battery outer casing body) to rated capacity is 5 cm 2 /Ah or more and the rated capacity is 3 Ah or more; and wherein the battery comprises a power generating element including a positive electrode, the positive electrode comprising: a positive electrode current collector; and a positive electrode active substance layer that is formed on a surface of the positive electrode current collector and has a positive electrode active substance containing a composite oxide comprising lithium and nickel and a spinel type lithium manganese composite oxide, wherein, when the average secondary particle diameter of the composite oxide comprising lithium and nickel as measured by laser diffraction is D50 [μm], the content ratio of the composite oxide comprising lithium and nickel in the positive electrode active substance layer is A [% by mass], the average secondary particle diameter of the spinel type lithium manganese composite oxide measured by laser diffraction is D50 [μm], and the content ratio of the spinel type lithium manganese composite oxide in the positive electrode active substance layer is B [% by mass], the positive electrode satisfies the following Mathematical Formula 1 and Mathematical Formula 2: 0.67 ≤ D 50 A / D 50 B ≤ 1.5 0.3 ≤ B / A + B ≤ 0.5; a positive electrode current collector; and a positive electrode active substance layer that is formed on a surface of the positive electrode current collector and has a positive electrode active substance containing a composite oxide comprising lithium and nickel and a spinel type lithium manganese composite oxide, wherein, when the average secondary particle diameter of the composite oxide comprising lithium and nickel as measured by laser diffraction is D50 [μm], the content ratio of the composite oxide comprising lithium and nickel in the positive electrode active substance layer is A [% by mass], the average secondary particle diameter of the spinel type lithium manganese composite oxide measured by laser diffraction is D50 [μm], and the content ratio of the spinel type lithium manganese composite oxide in the positive electrode active substance layer is B [% by mass], the positive electrode satisfies the following Mathematical Formula 1 and Mathematical Formula 2: 0.67 ≤ D 50 A / D 50 B ≤ 1.5 0.3 ≤ B / A + B ≤ 0.5; wherein the composite oxide comprising lithium and nickel has a composition represented by General Formula: Li a Ni b Mn c Co d M x O 2, with the proviso that, in the formula, a, b, c, d, and x satisfy 0.9 ≤ a ≤ 1.2, 0.44 ≤ b ≤ 0.51, 0.27 ≤ c ≤ 0.31, 0.19 ≤ d ≤ 0.26, 0 ≤ x ≤ 0.3, and b + c + d = 1, M represents at least one selected from a group consisting of Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, and Cr.