To solve the problem that much useless circuit design including assembling two parallel FETs in a protecting circuit is required for keeping the resistance of a whole battery pack lower if a PTC element is used as an overheat protecting element for the battery pack, and that a control IC of the protecting circuit detects an overdischarging condition of zero battery voltage and shuts off a current passage to prohibit the use until the next charge if the protecting circuit is formed using the conventional control IC and the thermal protector 2 is used as the overheat protecting element when the thermal protector 2 instantaneously breaks with dropping impact or the like.

A thermal protector 2 is used in the overheat protecting element and a control IC is used in a protecting circuit 3 for detecting the battery voltage during overcharge to shut off the current passage and reset it upon recovering the voltage.