To solve a problem that a waterproof defective is generated under severe circumstances of a long period use, a high temperature, a low temperature and high humidity in a portable wristwatch having a waterproof structure using a rubber packing.

A waterproof rubber packing is manufactured by adding a foaming agent to a rubber material, and melding it, and impregnating the obtained porous body in liquid oil or grease. The rubber packing containing oil or grease has excellent barrier property for gas such as steam, waterproof and rubber characteristics. Since the surface of the rubber packing is always covered with oil or grease, the packing has excellent lubricity. As a result, excellent waterproof can maintained for a long period.