A signal processing board includes a six-layer substrate. A plurality of signal transmission planes are formed in a first layer, a third layer, a fourth layer, and a sixth layer. A first ground plane is formed in a second layer. A first power supply plane is formed in a fifth layer and electrically connected to the first semiconductor element. A second power supply plane is formed in the fifth layer and electrically connected to the second semiconductor element. A second ground plane is formed in the fifth layer. A first bypass capacitor is electrically connected to the first power supply plane and the second ground plane. A second bypass capacitor is electrically connected to the second power supply plane and the second ground plane.