A thin metal plate and two prismatic-bar-shaped metal members that are parallel to each other are alternately and repeatedly stacked, and the stack is sandwiched between two thick metal plates. Each contact surface is bonded to the counterpart surface by diffusion bonding to form an integrated multilayer body. The multilayer body is cut at predetermined intervals at planes perpendicular to the thin metal plates, whereby a grid-like electrode is completed, with the thin metal plates serving as crosspieces and the metal members serving as spacers for defining a gap which serves as openings.