To provide a protecting device of secondary battery that meets the basic functions of overcharge, overcurrent protections of a secondary battery, and achieves simplified circuits, reduced cost, and reduced size.

A protective device has a switching circuit, connected to a battery charging circuit that feeds electricity to secondary batteries, and the like, connected in parallel relative to the charging circuit, and forces the device into conduction by the voltages that are not smaller than the threshold voltage, a first resistance element connected to the switching circuit in series, and a second resistance element connected to the charging circuit in series, the first resistance element and the second resistance element utilizing a protecting component comprising at least one heating elements. If the first resistive element of the heating element is Rb, the second resistance element of the heating element is Rb, the voltage protection of the secondary battery is X, and the voltage of the charge circuit is Y, the inequality equation Ra/Rb>(Y-X)/X is obtained.