A circuit substrate includes a core substrate having cavity penetrating through the core substrate, a metal block accommodated in the cavity of the core substrate, a first build-up layer including an insulating layer and laminated on first side of the core substrate such that the first build-up layer is covering the cavity on the first side of the core substrate, and a second build-up layer including an insulating layer and laminated on second side of the core substrate such that the second build-up layer is covering the cavity on the second side of the core substrate, and a filling resin filling gap formed between the cavity and block positioned in the cavity of the core substrate. The block has roughened surfaces such that the roughened surfaces are in contact with the insulating layers in the first and second build-up layers on the first and second sides of the core substrate.