A lithium ion secondary battery comprising: an electrode assembly formed by rolling up a positive electrode, a separator, and a negative electrode; an organic electrolyte solution; a positive electrode lead connected to the positive electrode; a negative electrode lead connected to the negative electrode; and an overcharge preventer, which are incorporated in a battery can sealed with a cap body, and comprising: a positive electrode terminal fixed to the cap body, and connected to the positive electrode lead; and a negative electrode terminal fixed to the cap body and connected to the negative electrode lead, wherein the overcharge preventer comprises: a voltage detector configured to detect a voltage equal to or higher than a predetermined value between the positive and negative electrodes; a fuse body provided on the positive electrode lead or the negative electrode lead; a switch unit; and a driver configured to drive the switch unit, the fuse body and the switch unit are connected in series to constitute a short circuit for the electrode assembly, and when the voltage between the positive and negative electrodes reaches a predetermined value or higher in the case of charging, the driver is activated by a signal from the voltage detector to close the switch unit, and melt down the fuse body by energy accumulated in the electrode assembly, the switch unit comprises a part of the positive electrode lead and a part of the negative electrode lead opposed to each other, and an insulating resin film placed between the part of the positive electrode lead and the part of the negative electrode lead, and a push force is always applied in a direction of bringing the both leads into contact with each other between the part of the positive electrode lead and the part of the negative electrode lead, and the insulating resin film has one end fixed, and the other end with a pull force applied thereto by a spring material. wherein the overcharge preventer comprises: a voltage detector configured to detect a voltage equal to or higher than a predetermined value between the positive and negative electrodes; a fuse body provided on the positive electrode lead or the negative electrode lead; a switch unit; and a driver configured to drive the switch unit, the fuse body and the switch unit are connected in series to constitute a short circuit for the electrode assembly, and when the voltage between the positive and negative electrodes reaches a predetermined value or higher in the case of charging, the driver is activated by a signal from the voltage detector to close the switch unit, and melt down the fuse body by energy accumulated in the electrode assembly, the switch unit comprises a part of the positive electrode lead and a part of the negative electrode lead opposed to each other, and an insulating resin film placed between the part of the positive electrode lead and the part of the negative electrode lead, and a push force is always applied in a direction of bringing the both leads into contact with each other between the part of the positive electrode lead and the part of the negative electrode lead, and the insulating resin film has one end fixed, and the other end with a pull force applied thereto by a spring material.