A supercooling apparatus of a simultaneous cooling and heating type multiple air conditioner comprises a liquid pipe header connected to outdoor heat exchangers, a plurality of branch liquid pipes branched from the liquid pipe header and connected to a plurality of indoor heat exchangers, respectively, and a supercooling mechanism mounted at least one of the branch liquid pipes for cooling refrigerant flowing to the indoor heat exchangers. The liquid refrigerant discharged from the heating-side indoor heat exchangers is supercooled by supercooling heat exchangers, and is then introduced into the cooling-side indoor heat exchangers. Consequently, noise generated from the cooling-side heat exchangers during the simultaneous cooling and heating operation is reduced, and cooling capacity of the multi air conditioner is improved.