1.  Field of the Invention
The present invention relates to a portable terminal that is rotatably openable and used in mobile communication such as of so-called clamshell, slide or swivel portable telephone or a portable information instrument, and a built-in antenna. 
2.  Description of the Related Art
A portable terminal has been diversified in function.  On the other hand, users demand a smaller and lighter portable terminal from the viewpoint of portability.  As a technology for miniaturizing and reducing the thickness of a terminal to satisfy such a demand, a miniaturization technology of an antenna a physical magnitude of which determines performance upon communicating externally is important. 
As a technology for miniaturizing an antenna, a technology in which, for example, a conductor pattern is partially disposed on a chassis that incorporates a printed circuit board on which a wireless circuit is mounted, and the conductor pattern and the wireless circuit are pressure-bonded with a plate spring made of a plate or a spring connector to mutually connect electrically to function as an antenna has been proposed (for example, Jpn.  Pat.  Appln.  KOKAI Publication No.  2005-295578). 
By the way, the chassis is generally made of plastic, and a method where a metal that becomes a conductor pattern is plated by electroless plating on a chassis made of such a material has been known (for example, Jpn.  Pat.  Appln.  KOKAI Publication No.  5-44047).  In the electroless plating method, a material to be plated is treated with a solution containing trivalent iron ions and divalent metallic ions capable of forming a ferrite magnetic body therewith, followed by neutralizing to precipitate ferrite on a surface thereof, further followed by treating in an electroless plating bath. 
However, in an antenna formed by an electroless plating method according to Patent Document 2, when the antenna is used under high-temperature and high-humidity conditions for a long time, there is a problem in that an impurity such as water penetrates through pinholes into the conductor pattern to tend to cause corrosion of the plating.