A power module substrate allows prompt heat dissipation from a semiconductor device and avoids separation of a ceramic plate and a copper plate at their joint interface and cracks in the ceramic plate. A power module substrate for mounting a semiconductor device includes a ceramic plate, a copper circuit plate on a main surface of the ceramic plate, and a heat dissipation copper plate on a surface of the ceramic plate opposite to the main surface. The copper circuit plate includes at least one first copper circuit plate and at least one second copper circuit plate different from the first circuit board. The first copper circuit plate includes a first portion on which the semiconductor device is mountable, and a second portion outward from the first portion and surrounding the first portion and thinner than the first portion.