A circuit-constituting unit forming a distribution circuit or the like in a vehicle. The circuit-constituting unit includes a plurality of bus bars for constituting a power circuit; a semiconductor switching device provided in the power circuit; and a control circuit board. The bus bars are bonded to a surface of the control circuit board such that the bus bars are arranged to be generally coplanar with each other. The semiconductor switching device is mounted on both of the corresponding bus bars and the control circuit board. An opening may be formed through the control circuit board. In this case, one of terminals of the semiconductor switching device may be connected to a surface of the control circuit board facing away from the surface to which the bus bars are bonded. The other terminals may be connected respectively to the bus bar through the opening.