A multilayer circuit substrate obtained by alternately stacking conductor layers and insulator layers. The conductor layers include a core layer having a greater thickness than any of the other conductor layers and located in an inner layer of the multilayer circuit substrate. A first conductor layer facing the core layer through an insulator layer has first signal wires that transmit high frequency signals, and through-holes are formed in the core layer along the first signal wires in a location facing the first signal wires.