A combustor configured for mixing compressed air supplied from a compressor with fuel and combusting the compressed air-fuel mixture, the combustor comprising: an outer can into which the fuel is introduced from the outside; an outer head disposed on a front side of the outer can; an inner can disposed inside of the outer can so that the compressed air flows between the inner can and the outer can, the inner can having a combustion chamber for combusting a mixture of the fuel and the compressed air therein; and an inner head disposed in front of the inner can to mix the fuel and the compressed air and supply the mixture into the inner can, the inner head comprising: a head plate covering a front side of the inner can; and a plurality of nozzle assemblies disposed in front of the head plate to mix the fuel and the compressed air and supply the mixture rearwards, the nozzle assemblies each comprising: a nozzle head formed in a hollow plate shape so that the fuel is introduced from a front side thereof into the nozzle head; and a plurality of nozzles each arranged such that a front side thereof is coupled to the nozzle head and a rear side thereof is coupled to the head plate so as to mix the fuel and the compressed air and supply the mixture rearwards, wherein each nozzle comprises a nozzle decreasing part disposed inside of the nozzle head and having a diameter decreasing toward a rear side thereof, with fuel holes formed through which fuel introduced into the each nozzle is supplied into the nozzle decreasing part, so that each nozzle has a shape with a diameter decreasing and increasing toward the rear side thereof. an outer can into which the fuel is introduced from the outside; an outer head disposed on a front side of the outer can; an inner can disposed inside of the outer can so that the compressed air flows between the inner can and the outer can, the inner can having a combustion chamber for combusting a mixture of the fuel and the compressed air therein; and an inner head disposed in front of the inner can to mix the fuel and the compressed air and supply the mixture into the inner can, the inner head comprising: a head plate covering a front side of the inner can; and a plurality of nozzle assemblies disposed in front of the head plate to mix the fuel and the compressed air and supply the mixture rearwards, the nozzle assemblies each comprising: a nozzle head formed in a hollow plate shape so that the fuel is introduced from a front side thereof into the nozzle head; and a plurality of nozzles each arranged such that a front side thereof is coupled to the nozzle head and a rear side thereof is coupled to the head plate so as to mix the fuel and the compressed air and supply the mixture rearwards, wherein each nozzle comprises a nozzle decreasing part disposed inside of the nozzle head and having a diameter decreasing toward a rear side thereof, with fuel holes formed through which fuel introduced into the each nozzle is supplied into the nozzle decreasing part, so that each nozzle has a shape with a diameter decreasing and increasing toward the rear side thereof. a head plate covering a front side of the inner can; and a plurality of nozzle assemblies disposed in front of the head plate to mix the fuel and the compressed air and supply the mixture rearwards, the nozzle assemblies each comprising: a nozzle head formed in a hollow plate shape so that the fuel is introduced from a front side thereof into the nozzle head; and a plurality of nozzles each arranged such that a front side thereof is coupled to the nozzle head and a rear side thereof is coupled to the head plate so as to mix the fuel and the compressed air and supply the mixture rearwards, wherein each nozzle comprises a nozzle decreasing part disposed inside of the nozzle head and having a diameter decreasing toward a rear side thereof, with fuel holes formed through which fuel introduced into the each nozzle is supplied into the nozzle decreasing part, so that each nozzle has a shape with a diameter decreasing and increasing toward the rear side thereof. a nozzle head formed in a hollow plate shape so that the fuel is introduced from a front side thereof into the nozzle head; and a plurality of nozzles each arranged such that a front side thereof is coupled to the nozzle head and a rear side thereof is coupled to the head plate so as to mix the fuel and the compressed air and supply the mixture rearwards, wherein each nozzle comprises a nozzle decreasing part disposed inside of the nozzle head and having a diameter decreasing toward a rear side thereof, with fuel holes formed through which fuel introduced into the each nozzle is supplied into the nozzle decreasing part, so that each nozzle has a shape with a diameter decreasing and increasing toward the rear side thereof.