A first connector is soldered to a first board. A second connector is soldered to a second board. A third connector and a fourth connector are soldered to a relay board. The third connector is connected to the first connector via a first power line. The fourth connector is connected to the second connector via a second power line. A surface to which the first connector is soldered on the first board is opposite to a surface to which the first connector is attached on the first board. A surface to which the second connector is soldered on the second board is a surface to which the second connector is attached on the second board. Only surface-mount devices are used as electronic components soldered to the second board.