A power module comprising: a power module substrate provided with a circuit layer on one surface of an insulating layer; and a semiconductor element bonded onto the circuit layer, wherein the entire circuit layer corresponds to a copper layer composed of copper or a copper alloy or the circuit layer includes an aluminum layer and a copper layer composed of copper or a copper alloy on one surface of the aluminum layer and provided on the surface of the circuit layer, onto which the semiconductor element is bonded wherein the aluminum layer is formed by bonding a rolled plate of aluminum having a purity of 99.99% by mass or more to the insulating layer, a solder layer formed by using a solder material is formed between the circuit layer and the semiconductor element, an alloy layer consisting of Sn and inevitable impurities as a balance, 0.5% by mass or more and 10% by mass or less of Ni, and 30% by mass or more and 40% by mass or less of Cu is formed at an interface between the solder layer and the circuit layer, a thickness of the alloy layer is set to be within a range of 2 μm or more and 20 μm or less, a thermal resistance increase rate is less than 10% after loading power cycles 100,000 times under the condition where an energization time is 5 seconds and a temperature difference is 80°C in a power cycle test, the alloy layer includes an intermetallic compound composed of (Cu, Ni) 6 Sn 5, the solder material is composed of a Sn-0.1% by mass to 4% by mass Cu-0.01% by mass to 1% by mass Ni alloy, a thickness of the solder layer is set to be within a range of 50 μm or more and 200 μm or less, an intermetallic compound layer composed of an intermetallic compound Cu 3 Sn of Cu and Sn is formed on the surface of the circuit layer and the solder layer is provided on the intermetallic compound layer in a laminated arrangement, and a thickness of the intermetallic compound layer is set to 0.8 μm or less, wherein the composition of the alloy layer, the thickness of the alloy layer, the presence of the intermetallic compound composed of (Cu, Ni) 6 Sn 5, the thickness of the intermetallic compound layer, the thermal resistance increase rate are determined utilizing the measurement methods described in the description. a power module substrate provided with a circuit layer on one surface of an insulating layer; and a semiconductor element bonded onto the circuit layer, wherein the entire circuit layer corresponds to a copper layer composed of copper or a copper alloy or the circuit layer includes an aluminum layer and a copper layer composed of copper or a copper alloy on one surface of the aluminum layer and provided on the surface of the circuit layer, onto which the semiconductor element is bonded wherein the aluminum layer is formed by bonding a rolled plate of aluminum having a purity of 99.99% by mass or more to the insulating layer, a solder layer formed by using a solder material is formed between the circuit layer and the semiconductor element, an alloy layer consisting of Sn and inevitable impurities as a balance, 0.5% by mass or more and 10% by mass or less of Ni, and 30% by mass or more and 40% by mass or less of Cu is formed at an interface between the solder layer and the circuit layer, a thickness of the alloy layer is set to be within a range of 2 μm or more and 20 μm or less, a thermal resistance increase rate is less than 10% after loading power cycles 100,000 times under the condition where an energization time is 5 seconds and a temperature difference is 80°C in a power cycle test, the alloy layer includes an intermetallic compound composed of (Cu, Ni) 6 Sn 5, the solder material is composed of a Sn-0.1% by mass to 4% by mass Cu-0.01% by mass to 1% by mass Ni alloy, a thickness of the solder layer is set to be within a range of 50 μm or more and 200 μm or less, an intermetallic compound layer composed of an intermetallic compound Cu 3 Sn of Cu and Sn is formed on the surface of the circuit layer and the solder layer is provided on the intermetallic compound layer in a laminated arrangement, and a thickness of the intermetallic compound layer is set to 0.8 μm or less, wherein the composition of the alloy layer, the thickness of the alloy layer, the presence of the intermetallic compound composed of (Cu, Ni) 6 Sn 5, the thickness of the intermetallic compound layer, the thermal resistance increase rate are determined utilizing the measurement methods described in the description.