A prismatic lithium secondary battery includes: a prismatic battery can having a bottom, a side wall, and an open top; an electrode assembly; a non-aqueous electrolyte; and a sealing plate covering the open top of the battery can that accommodates the electrode assembly and the non-aqueous electrolyte. The electrode assembly includes: a positive electrode; a negative electrode; and a porous heat-resistant layer and a separator that are interposed between the positive and negative electrodes. The side wall of the battery can has two rectangular main flat portions that are opposed to each other, and the thickness A of the porous heat-resistant layer and the thickness B of each of the main flat portions of the side wall satisfy the relation: 0.003≦A/B≦0.05.