A multilayer circuit substrate includes: a first conductor layer in which first transmission lines and a second transmission line are formed; a second conductive layer facing the first conductive layer through an insulating layer; and a third conductive layer that faces the second conductive layer through an insulating layer and that has a bypass line formed therein. The bypass line is electrically connected to the second transmission line of the first conductive layer through via conductors and such that the second transmission line and the first transmission lines intersect with each other. In the second conductive layer, a ground conductor is formed at least in a position that faces the bypass line, and the first transmission lines are made narrower at the intersection with the second transmission line than other portions.