A substrate for a power module (100) of the present invention includes a metal substrate (101), an insulating resin layer (102) provided on the metal substrate (101), and a metal layer (103) provided on the insulating resin layer (102). The insulating resin layer (102) includes a thermosetting resin (A) and inorganic fillers (B) dispersed in the thermosetting resin (A), a maximum value of a dielectric loss ratio of the insulating resin layer (102) at a frequency of 1 kHz and 100° C. to 175° C. is equal to or less than 0.030, and a change in a relative permittivity is equal to or less than 0.10.