A circuit board includes an insulating substrate, a first conductive layer on the insulating substrate, a second conductive layer on the first conductive layer, and a third conductive layer covering the first conductive layer and the second conductive layer. The first conductive layer has a surface provided on the surface of the insulating substrate, and a surface having a width smaller than a width of the above surface. In this circuit board, the conductive layers have small impedances even if a high-frequency signal flows in the conductive layers.