An insulating circuit board includes an insulating plate, a circuit board joined to a first surface of the insulating plate, and a metal plate joined to a second surface of the insulating plate. The circuit board is formed from an Al alloy having a purity of 99.98% or more or pure Al, and the metal plate is formed from an Al alloy having a purity of 98.00% or more and 99.90% or less. The thickness (a) of the circuit board is 0.2 mm or more and 0.8 mm or less, the thickness (b) of the metal plate is 0.6 mm or more and 1.5 mm or less, and the thicknesses satisfy the expression of a/b≦1. An insulating circuit board having a cooling sink includes cooling sink joined via a second solder layer. The second solder layer contains Sn as its main component, and has a Young's modulus, 35 GPa or more, a 0.2% proof stress of, 30 MPa or more, and a tensile strength of, 40 MPa or more. The cooling sink is formed from, pure Al or an Al alloy.