A compact ventilating system having an air purifying function is disclosed, which includes a case provided in the inside of a room; an air-supplying passage for penetrating a wall through the case under a ceiling, to introduce an outdoor air to the inside of the room by communicating the outside of the room, the case, and the inside of the room with one another; an air-discharging passage for penetrating the wall through the case under the ceiling, to discharge an indoor air to the outside of the room by communicating the inside of the room, the case, and the outside of the room with one another; an air-circulating passage for communicating the inside of the room with one point of the air-supplying passage in the case, to introduce the indoor air to the inside of the case, and to re-supply the introduced indoor air to the inside of the room; and a fan provided in the case, for providing the outdoor air introduced to the case through the air-supplying passage to the inside of the room, or re-supplying the indoor air introduced to the case through the air-circulating passage to the inside of the room.