Prismatic secondary battery

A prismatic secondary battery is a prismatic lithium-ion battery (20) including a stack-type electrode assembly (10) in which square positive and negative electrode plates (2)(3) are stacked with separators (13) interposed therebetween. The positive electrode plates are arranged inside a bag-like separator. The width of the separator protruding from an end portion of each positive electrode plate on a non-joined side of the separator is greater than that of the separator protruding from an end portion of the positive electrode plate on a joined side of the separator. The heat-shrinkage rate of the separator in a direction vertical to the non-joined side is greater than that of the separator in a direction parallel to the non-joined side. Short circuiting between the positive and negative electrode plates due to heat shrinkage or rupture of the separator is prevented even when abnormal heat generation occurs in the battery.