A simple configuration for cooling comprises a front-back air supply and exhaust system. A first circuit board is located in front of a relay circuit board. A cooling unit and a second circuit board are placed behind the relay circuit board in a chassis. A first air passage allows intake air through an intake hole in a front side of the first circuit board unit to pass through the first circuit board and then through an opening in the relay circuit board to the cooling unit. A second air passage allows intake air through an intake hole in a front face of the chassis to pass through a lateral side of the first circuit board and then through a vent hole in a partition provided at the lateral side of the first circuit board. The second circuit board is placed in the second air passage.