A second printed wiring board is opposed to a surface of the first printed wiring board. A support member supports the first and second printed wiring boards. A first connector is mounted on the first printed wiring board. A second connector is mounted on the second printed wiring board. A wiring connects the first printed wiring board to the second printed wiring board. The first and second connectors are separately mounted on the first and second printed wiring boards, respectively. The sizes of the first and second printed wiring boards can be reduced as compared with the case where connectors are arranged in a row on a single printed wiring board, for example. This results in a reduction in the size of the printed circuit board unit. A large number of the printed circuit board units can be coupled to the back panel.