To realize the high density inerconnection of a circuit board.

Electric circuit parts 50 and 52, composed of multilayer hard boards are provided on the middle part of a strip connection terminal leading- out flexible board 71 so as to hold the flexible board between them. Wiring patterns formed on the flexible board are connected to the multilayer electric circuit parts with through-holes. Electronic components 88 and 89 are mounted on one of or both of the electric circuit parts provided on both the sides of the flexible board. As the electric circuit parts and the flexible boards are connected to each other with the through-holes, it is not necessary to provide connectors and land patterns on the boards themselves of which the electric circuit parts are composed. With this constitution, the mounting areas for electronic components, etc., can be assured, so that the circuit board whose interconnection density is higher than the circuit board of a conventional constitution can be devised.