Resin multilayer substrate and circuit module

A resin multilayer substrate comprising:

• at least one resin insulating layer;
• a first conductor layer stacked on the resin insulating layer;
• a second conductor layer stacked on the resin insulating layer on a side opposite to the first conductor layer;
• and
• an interlayer connection conductor penetrating the resin insulating layer in a stacking direction of the first conductor layer and the second conductor layer and connecting the first conductor layer and the second conductor layer;
• wherein
• the interlayer connection conductor includes a first portion and a second portion located closer to the second conductor layer than the first portion;
• the first portion has lower conductivity than the second portion;
• the first conductor layer is not in contact with the second portion;
• the second conductor layer is in contact with the second portion;
• and
• when viewed in cross section in a direction perpendicular or substantially perpendicular to the stacking direction, a distance between two corner portions at which a distal end portion of the second portion is most separated in the direction perpendicular or substantially perpendicular to the stacking direction is longer than a distance between two corner portions at which a proximal end portion of the second portion is in contact with the second conductor layer, and a portion of the first portion is closer to the second conductor layer than a straight line connecting the corner portions of the distal end portion of the second portion.