A laundry treating apparatus comprising: a first laundry treating machine including: a first cabinet; and a first drum rotatably disposed in the first cabinet; a second laundry treating machine including: a second cabinet vertically disposed with respect to the first cabinet; and a second drum rotatably disposed in the second cabinet; wherein the laundry treating apparatus comprises: a third laundry treating machine including a third cabinet disposed at a lateral side of the first cabinet and the second cabinet, the third cabinet comprising an inner space for receiving laundry; a heating device disposed at a lateral side of the third cabinet, the heating device configured to heat air supplied into the inner space of the third cabinet; and a duct system which includes passages of hot air heated by the heating device, and wherein the duct system comprises: a heat exchange channel in which air is heated by the heating device; a supply air duct connecting the heating device and the third laundry treating machine; and an exhaust air duct through which air supplied to the third laundry treating machine flows into the heat exchange channel. a first laundry treating machine including: a first cabinet; and a first drum rotatably disposed in the first cabinet; a first cabinet; and a first drum rotatably disposed in the first cabinet; a second laundry treating machine including: a second cabinet vertically disposed with respect to the first cabinet; and a second drum rotatably disposed in the second cabinet; wherein the laundry treating apparatus comprises: a third laundry treating machine including a third cabinet disposed at a lateral side of the first cabinet and the second cabinet, the third cabinet comprising an inner space for receiving laundry; a heating device disposed at a lateral side of the third cabinet, the heating device configured to heat air supplied into the inner space of the third cabinet; and a duct system which includes passages of hot air heated by the heating device, and wherein the duct system comprises: a heat exchange channel in which air is heated by the heating device; a supply air duct connecting the heating device and the third laundry treating machine; and an exhaust air duct through which air supplied to the third laundry treating machine flows into the heat exchange channel. a second cabinet vertically disposed with respect to the first cabinet; and a second drum rotatably disposed in the second cabinet; wherein the laundry treating apparatus comprises: a third laundry treating machine including a third cabinet disposed at a lateral side of the first cabinet and the second cabinet, the third cabinet comprising an inner space for receiving laundry; a heating device disposed at a lateral side of the third cabinet, the heating device configured to heat air supplied into the inner space of the third cabinet; and a duct system which includes passages of hot air heated by the heating device, and wherein the duct system comprises: a heat exchange channel in which air is heated by the heating device; a supply air duct connecting the heating device and the third laundry treating machine; and an exhaust air duct through which air supplied to the third laundry treating machine flows into the heat exchange channel. a third laundry treating machine including a third cabinet disposed at a lateral side of the first cabinet and the second cabinet, the third cabinet comprising an inner space for receiving laundry; a heating device disposed at a lateral side of the third cabinet, the heating device configured to heat air supplied into the inner space of the third cabinet; and a duct system which includes passages of hot air heated by the heating device, and wherein the duct system comprises: a heat exchange channel in which air is heated by the heating device; a supply air duct connecting the heating device and the third laundry treating machine; and an exhaust air duct through which air supplied to the third laundry treating machine flows into the heat exchange channel. a heat exchange channel in which air is heated by the heating device; a supply air duct connecting the heating device and the third laundry treating machine; and an exhaust air duct through which air supplied to the third laundry treating machine flows into the heat exchange channel.