An safety valve (14) for discharging decomposition gas of an electrolytic solution, which is generated inside a battery when the battery is overcharged, to an outside of the battery is provided in a lithium ion secondary battery (10). By setting a valve opening pressure of the safety valve (14) to 0.54 MPa, the safety valve (14) is opened approximately 95 seconds before an inside short-circuit occurs when the battery is overcharged, and the decomposition gas of the electrolytic solution is discharged outside the battery promptly.