A power-module substrate unit having at least one power-module substrate including one ceramic substrate, a circuit layer formed on one surface of the ceramic substrate, and a metal layer formed on another surface of the ceramic substrate, and a heat sink on which the metal layer of the power-module substrate is bonded, in which the metal layer is made of an aluminum plate having purity of 99.99 mass % or higher; the heat sink is made of an aluminum plate having purity of 99.90 mass % or lower; and the circuit layer has a stacking structure of a first layer made of an aluminum plate having the purity of 99.99 mass % or higher and being bonded to the ceramic substrate and a second layer made of the aluminum plate having the purity lower than 99.90 mass % and being bonded on a surface of the first layer.