A method for producing an unsaturated nitrile by subjecting propane to a vapor-phase catalytic ammoxidation reaction using a fluidized bed reactor in the presence of a composite oxide catalyst containing Mo, V, and Nb, wherein the method comprises a step of: continuously or intermittently adding a tungsten compound selected from a salt of tungsten, tungsten trioxide (WO 3), tungsten dioxide, tungstate, ammonium metatungstate, tungstosilicic acid, silicotungstomolybdic acid and vanadotungstosilicic acid into the fluidized bed reactor to adjust the molar ratio (W/Mo ratio) of the tungsten contained in the tungsten compound to the molybdenum contained in the composite oxide catalyst that exist within the fluidized bed reactor so that the molar ratio is in the range of 0.0001 to 0.1, wherein the W/Mo molar ratio of tungsten contained in the tungsten compound to molybdenum contained in the composite oxide catalyst within the fluidized bed reactor is determined by determining the number of moles of molybdenum contained in the composite oxide catalyst and the number of moles of tungsten contained in the tungsten compound during the reaction, wherein the concentration of molybdenum in the composite oxide catalyst is determined by extracting part of the composite oxide catalyst from the fluidized bed reactor followed by fluorescent X-ray analysis, wherein the particle size of the tungsten compound added is different from the particle size of the composite oxide catalyst filling the reactor in advance, and wherein a part of the composite oxide catalyst is extracted from the fluidized bed reactor during the reaction for determining the number of moles of molybdenum contained in said composite oxide catalyst by fluorescent X-ray analysis in order to determine as described in the description the amount of tungsten compound to be added into the fluidized bed reactor for adjusting the W/Mo molar ratio, and wherein the method further comprises a step of adding a molybdenum compound into the fluidized bed reactor, wherein the molybdenum compound is selected from the group consisting of ammonium heptamolybdate [(NH 4) 6 Mo 7 O 24 ·4H 2 O], molybdenum trioxide [MoO 3], phosphorus molybdate [H 3 PMo 12 O 40], silicon molybdate [H 4 SiMo 12 O 40], and molybdenum pentachloride [MoCl 5]. continuously or intermittently adding a tungsten compound selected from a salt of tungsten, tungsten trioxide (WO 3), tungsten dioxide, tungstate, ammonium metatungstate, tungstosilicic acid, silicotungstomolybdic acid and vanadotungstosilicic acid into the fluidized bed reactor to adjust the molar ratio (W/Mo ratio) of the tungsten contained in the tungsten compound to the molybdenum contained in the composite oxide catalyst that exist within the fluidized bed reactor so that the molar ratio is in the range of 0.0001 to 0.1, wherein the W/Mo molar ratio of tungsten contained in the tungsten compound to molybdenum contained in the composite oxide catalyst within the fluidized bed reactor is determined by determining the number of moles of molybdenum contained in the composite oxide catalyst and the number of moles of tungsten contained in the tungsten compound during the reaction, wherein the concentration of molybdenum in the composite oxide catalyst is determined by extracting part of the composite oxide catalyst from the fluidized bed reactor followed by fluorescent X-ray analysis, wherein the particle size of the tungsten compound added is different from the particle size of the composite oxide catalyst filling the reactor in advance, and wherein a part of the composite oxide catalyst is extracted from the fluidized bed reactor during the reaction for determining the number of moles of molybdenum contained in said composite oxide catalyst by fluorescent X-ray analysis in order to determine as described in the description the amount of tungsten compound to be added into the fluidized bed reactor for adjusting the W/Mo molar ratio, wherein the particle size of the tungsten compound added is different from the particle size of the composite oxide catalyst filling the reactor in advance, and wherein a part of the composite oxide catalyst is extracted from the fluidized bed reactor during the reaction for determining the number of moles of molybdenum contained in said composite oxide catalyst by fluorescent X-ray analysis in order to determine as described in the description the amount of tungsten compound to be added into the fluidized bed reactor for adjusting the W/Mo molar ratio, and wherein the method further comprises a step of adding a molybdenum compound into the fluidized bed reactor, wherein the molybdenum compound is selected from the group consisting of ammonium heptamolybdate [(NH 4) 6 Mo 7 O 24 ·4H 2 O], molybdenum trioxide [MoO 3], phosphorus molybdate [H 3 PMo 12 O 40], silicon molybdate [H 4 SiMo 12 O 40], and molybdenum pentachloride [MoCl 5].