A board-to-board connecting structure which adds no significant thickness to a single printed circuit board includes a first circuit board and a second circuit board. The first circuit board includes first circuit substrate, adhesive layer, and second circuit substrate. The first circuit substrate includes first base layer, first inner wiring layer with first pad, and first outer wiring layer defining a receiving space. The second circuit substrate includes insulating layer and two second outer wiring layers. A conductive via in the second circuit substrate connects the two second outer wiring layers. The second circuit board includes second base layer and also two third outer wiring layers each with a second pad. The second circuit board is laterally disposed in the receiving space and one second pad connects to the conductive via and the other to the first pad.