A method for manufacturing a prismatic lithium secondary battery having an electrode assembly having a separation film between a positive electrode plate and a negative electrode plate, the method including forming separation films, and positive and negative plates having a narrower width than the separation films, alternately interposing the positive and negative plates between the separation films and stacking the plates and films, hermetically sealing opposite sides of the separation films by heating along a lengthwise direction to form an electrode assembly, and immersing the electrode assembly in an electrolyte solution, putting the assembly into a case, and hermetically sealing the case.