In the power module substrate unit (51) of the present invention, the circuit layer (12) is composed of a plurality of small circuit layers (12S), the ceramic substrate layer (11) is composed of at least one piece, and the metal layer (13) is composed of at least one piece, The small circuit layer (12S) is a stacked structure, and the stacked structure has a first aluminum layer (15) bonded to one side of the ceramic substrate layer (11) and a first copper layer (15) bonded to the first aluminum layer (15) by solid phase diffusion layer (16), the metal layer (13) is formed by the same material as the first aluminum layer (15), the heat dissipation plate (30) is formed by copper or copper alloy, and the metal layer (13) and the heat dissipation plate (30) are diffused in solid phase For bonding, set the thickness of the first copper layer (16) as t1 (mm), and set the bonding area as A1 (mm 2 ), set the yield strength as σ1(N / mm 2 ), the thickness of the heat dissipation plate (30) on the position where the metal layer is bonded is set as t2, and the joint area is set as A2 (mm 2 ), set the yield strength as σ2(N / mm 2 ), the ratio (t1×A1×σ1) / (t2×A2×σ2) is 0.80 or more and 1.20 or less.