A lithium secondary battery and a method for manufacturing the lithium secondary battery includes the forming of an electrode assembly by interposing a separator between a positive electrode and a negative electrode, accommodating the electrode assembly into a battery case, injecting an electrolytic solution into the battery case, and forming the battery. The act of sealing the battery is by reducing the internal pressure of the battery to be maintained at a certain range of pressure for a certain period of time, is further provided after injecting an electrolytic solution or after forming the battery. The lithium second battery can prevent deformation of an external shape of a battery due to swelling under a high-temperature condition and can prevent deterioration in performance of a battery.