NOVELTY - A square nonaqueous electrolyte secondary battery (10) has flat-shaped winding electrode component (14) having 0.1-0.9 of unpressed portion obtained by winding positive electrode (11) formed using positive mix layer, and negative electrode (12) formed using negative mix layer, through separator (13), and nonaqueous electrolyte containing electrolyte salt in nonaqueous solvent. The nonaqueous electrolyte contains additive containing 1,3-dioxane and aromatic compound chosen from compound having cycloalkyl benzene compound and quaternary carbon adjacent to benzene ring.    USE - Square nonaqueous electrolyte secondary battery e.g. lithium-ion secondary battery. Uses include but are not limited to hybrid electric vehicle, mobile telephone, portable personal computer and portable music player.    ADVANTAGE - The reaction of additive agent in nonaqueous electrolyte of square nonaqueous electrolyte secondary battery is stimulated during overcharging, and the stability during overcharging is improved.    DETAILED DESCRIPTION - A square nonaqueous electrolyte secondary battery (10) has flat-shaped winding electrode component (14) obtained by winding positive electrode (11) formed using fixed thickness of positive mix layer, and negative electrode (12) formed using fixed thickness of negative mix layer, through separator (13), and nonaqueous electrolyte containing electrolyte salt in nonaqueous solvent. The nonaqueous electrolyte contains additive containing 1,3-dioxane and aromatic compound chosen from compound having cycloalkyl benzene compound and quaternary carbon adjacent to benzene ring. A 0.1-0.9 of unpressed portion is formed in the flat-shaped winding electrode component.    DESCRIPTION OF DRAWING(S) - The drawing shows a perspective view of the square nonaqueous electrolyte secondary battery.    Square nonaqueous electrolyte secondary battery (10)    Positive electrode (11)    Negative electrode (12)    Separator (13)    Flat-shaped winding electrode component (14)