Disclosed is a combined ventilating and air conditioning system, comprising: a first air duct for guiding outdoor air to the indoor spaces, the first air duct being capable of opening and closing; a second air duct for guiding indoor air to outdoors, wherein the second air duct is capable of opening and closing and installed in such a manner that part of the second air duct intersects with the first air duct; a third air duct for guiding the indoor air to the outdoors, the third air duct being capable of opening and closing; a regenerative heat exchanger, being provided in an intersection between the first and second air ducts, for exchanging heat between the indoor air and the outdoor air passing through the intersection; and blowing fans for controlling airflows, being provided in the first air duct, in the second air duct, and in the third air duct, respectively. The compressor mounted on the air conditioner is provided in the third air duct. The second heat exchanger is provided nearby the third air duct through which the indoor air is intaken, and the first heat exchanger is provided nearby the first air duct through which the outdoor air is emitted from the first air duct to the indoor spaces.