A circuit substrate (1) includes an insulating substrate (2), metal circuit plates (3) that are joined to one main surface of the insulating substrate (2), and a metal heat dissipation plate (4) that is joined to a main surface which is on the side opposite the one main surface of the insulating substrate (2). The thickness of the heat dissipation plate (4) is at least 3.75 times the thickness of the metal circuit plates (3). The particle diameter of metal particles contained in the heat dissipation plate (4): is smaller than the particle diameter of the metal particles contained in the metal circuit plate (3); and becomes smaller the greater the distance from the opposite-side main surface of the insulating substrate (2).