A circuit board has a base layer composed of multiple layers, and the base layer includes a connector to which a wire harness carrying a signal is coupled, the connector being fixed to a slot for mounting, and a plurality of GND plane patterns arranged in a plurality of layers and electrically coupled to each other through at least one via. Each one of the multiple layers of the base layer includes one of the plurality of GND plane patterns. In and around an area in which the connector is disposed, each one of the plurality of GND plane patterns is provided for a respective one of the multiple layers. The at least one via is arranged around the area in which the connector is disposed. The base layer includes a frame, and at least one via is arranged in the frame.