A circuit device having superior mechanical strength at the interface between a circuit board and heat sink and superior efficiency for radiating heat from a circuit element to the heat sink through the circuit board. The circuit device includes the metal-based insulation board for installing the circuit element, and the heat sink, over which the insulation board is installed with a paste arranged therebetween. The insulation board has a projection arranged on the surface facing the heat sink along a peripheral portion. At least part of the projection contacts the heat sink through the paste layer.