To provide a nonaqueous electrolyte secondary battery with high reliability capable of avoiding nonaqueous electrolyte from being squeezed from an electrode group even under a circumstance not allowing dimensional change of a battery case.

The battery is composed of the electrode group, nonaqueous electrolyte, and a nearly square-shaped battery case 4 housing the electrode group and the nonaqueous electrolyte. The thickness α, width β, and height γ of the battery case fulfills relation of α<β≤γ. The electrode group is composed of a cathode 1, an anode 2, and a porous heat resistant layer 3 laid between the cathode and the anode. The cathode contains a cathode activator layer, the anode contains an anode activator layer, and the ratio of volume of cavity contained in a prescribed area of the porous heat resistant layer is 0.18 to 1.117 ml/Ah. The area of the prescribed area is same with that of the cathode activator layer, and porosity of the porous heat resistant layer is 35 to 85%.

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