PURPOSE: To protect a lithium battery during over-current by soldering one end of a lead wire to which force is applied in such a manner as to cause it to be separated from the electrode and cutting off the circuit when the temperature exceeds the melting point of solder.

CONSTITUTION: Lithium batteries 10A and 10B are installed in a case 30 with metallic members 32 and 34 interposed. An opening 36 is formed in the case 30. A lead wire 42 is used to connect electrodes 38 and 40. One end 42A of the lead wire 42 is fixed to the electrode 38 by spot welding and force is applied to the other end 2B in such a manner as to cause the other end 2B to be separated from the electrode 40 when external force is not applied to the end 2B. The end 2B is fixed to the electrode 40 by soldering against the force applied by low- molting-point solder. Because of the above constitution, the temperature rise of the batteries 10A and 10B caused by overcurrent causes melting of the low melting point solder to separate the lead wire 42 from the electrode 40 thereby cutting off the circuit. As a result, the safety of the battery can be secured without providing any thermal fuses.