First and second semiconductor devices and first and second bypass circuits are mounted on a printed wiring board. The first bypass circuit and the second bypass circuit are provided closer to the first semiconductor device and to the second semiconductor device, respectively. The first bypass circuit has one end connected to a power plane through a first power supply via and the other end connected to a ground plane through a first ground via. The second bypass circuit has one end connected to the power plane through a second power supply via and the other end connected to the ground plane through a second ground via. The ground plane has a slit between the connecting portions of the first and second ground vias to increase the impedance between the connecting portions of the first and the second ground vias. Thus, jitters caused by power supply noise can be reduced.