A circuit board includes a multilayer body including a main surface, a mounted conductor, and a signal conductor at an intermediate position in the lamination direction of the multilayer body, and a ground conductor on the main surface. The multilayer body includes a connection portion including a portion overlapping the mounted conductor and overlapping an external board joined via a conductive joint material through use of the mounted conductor, and a circuit portion. A first region, which is the region of a circuit portion of the ground conductor, includes opening holes and a second region, which is the region of a connection portion of the ground conductor, includes opening holes. The ratio of the opening area of the opening holes to that of the second region is larger than the ratio of the opening area of the opening holes to that of the first region.