A circuit board device suppress with a small number of terminal elements unwanted irradiation originating between a power supply layer and a ground layer, even when a configuration of the power supply layer and the ground layer on the circuit board is complex, and a design support device thereof. The circuit board device has a power supply layer and a ground layer disposed in opposition to one another. A dielectric is disposed between the power supply layer and the ground layer. A power supply surface is divided into two power supply surfaces and by a slit having a generally T-shaped configuration to form power supply surface edges. The power supply surface edges retain across a predetermined length L a characteristic impedance present between the power supply layer and the ground layer. A terminal load is connected to a terminal portion of the power supply surface edges.