PURPOSE: To prevent an explosion caused by the hydrogen gas in a battery when a catalyst plug is fitted to or removed from a cover by setting the surface resistance of the screw section of the catalyst plug or the inherent resistance value of the screw section to a specific Ωcm or below, and preventing the generation and retention of static electricity on the surface of the screw section.

CONSTITUTION: A cover 1 and a battery jar 2 of a battery A are made of an ABS resin having the inherent resistance value of 1016 Ωcm, and a catalyst container section 4 and a screw section 5 are made of a polypropylene resin having the inherent resistance value of 1017Ωcm. A gold thin film is formed by deposition on the surface of the screw section 5 of a battery B, a gold foil of about 100μm in thickness is wound on the surface of the screw section 5 of a battery C, and a screw section 5 molded with the carbon powder mixed with a polypropylene resin and having the inherent resistance of 106Ωcm is provided on a battery D. As a result of tests on these batteries, no electrostatic energy is generated at the screw section 5, or the generated electrostatic energy, if any, is smaller than the ignition energy of 0.02mj of hydrogen gas, the generation and retention of static electricity on the surface of the screw section 5 are prevented, and the occurrence of an explosion caused by the hydrogen gas in the battery can be prevented.