PURPOSE: To increase the bonding strength of thermoplastic resin to a metallic outer plate bonded by heat-sealing and the reliability of a sealed part by forming a composite layer, in which thermoplastic resin powder is dispersed in a metal layer by plating, in the sealing part of an outer plate.

CONSTITUTION: A composite layer (a) is formed by plating which uses a metal plating bath in which fine resin powder is dispersed. In the composite layer (a), resin powder (b) is dispersed in the metal and also studded on the surface. When the composite layer (a) is heat-sealed to a thermoplastic resin, the resin on the surface of an outer plate (d) is melt-bonded to a sealing resin. In addition, the resin on the surface of the composite layer (a) is anchored in the metal (c) and the bonding to the sealing resin is remarkably strengthened.