A plurality of film insulators having conductive patterns that are formed on surfaces and conductive vias that pass through the film insulators in the direction of thickness are stacked together and collectively subjected to pressure and heat to be formed as a single unit. On one outermost layer of the multilayer board that has been thus formed, a plurality of connection terminals are exposed to the outside, connection bumps of an LSI chip being secured to these connection terminals. On the outermost layer of the opposite side, a multiplicity of metal pads are provided, and a solder ball is secured on each metal pad to form a ball grid array (BGA) structure for connecting to a motherboard.