<p>An air-conditioning system includes a ceiling plenum disposed above a ceiling of a room; an air-conditioning machine disposed in an internal space of the room; a plurality of IT apparatus mounting racks disposed in a plurality of rows, with rear faces of the IT apparatus mounting racks in adjacent two rows opposed to each other; a upper partitioning board disposed between an upper face of the plurality of IT apparatus mounting racks and the ceiling; an end-partitioning board disposed between each of ends of a space between adjacent two rows of the plurality of IT apparatus mounting racks and the ceiling; and a ceiling opening portion provided in the ceiling above a warm-air space enclosed by the upper partitioning board and the end-partitioning board. The ceiling opening portion connects the warm-air space with the ceiling plenum.
</p>
