Insulating resin material and multilayer substrate

Provided is an insulating resin material that is capable of reducing the surface roughness of the surface of a cured product, and that is capable of increasing the bonding strength between a cured product and a metal layer when the metal layer is formed on the surface of the cured product. This insulating resin material comprises: a thermosetting resin; a curing agent; a first inorganic filler, the surface of which is treated using a first silane coupling agent; and a second inorganic filler, the surface of which is treated using a second silane coupling agent. When the absolute value of the difference between the SP value of the thermosetting resin that is the component constituting the largest proportion of the insulating resin material and the SP value of the organic groups of the first silane coupling agent is represented by SP(A) and the absolute value of the difference between the SP value of the thermosetting resin that is the component constituting the largest proportion of the insulating resin material and the SP value of the organic groups of the second silane coupling agent is represented by SP(B), (SP(A) - SP(B)) is 0.5-3.5.