To monitor the operation condition of a lithium secondary battery through the behavior of its container.

The container 1 of the lithium secondary battery 10 contains electrolyte. In case of short-circuit accident, etc., the electrolyte decomposes, causing gas generation and sharp increase in internal pressure. Temperature fluctuation due to charging and discharging a steady state also causes pressure fluctuation. A pressure sensor 20 is arranged in the inside space of safety valve 7 of the lithium secondary battery 10. The internal pressure is detected by the pressure sensor 20, and the detected pressure is shown on a display 20. By monitoring the displayed pressure, it can be determined whether the condition of the lithium secondary battery is normal or abnormal.