A method for producing acrylonitrile by a propane ammoxidation reaction, using a composite oxide catalyst represented by the following formula: Mo 1 V a Nb b A c X d Z e O n wherein component A represents Sb; component X represents at least one element selected from the group consisting of W, Bi and Mn; component Z represents at least one element selected from the group consisting of La, Ce, Pr, Yb, Y, Sc, Sr, and Ba; a, b, c, d, e, and n each represent an atomic ratio of the corresponding element per Mo atom; a is in the range of 0.01 ≦ a ≦ 1; b is in the range of 0.01 ≦ b ≦ 1; c is in the range of 0.01 ≦ c ≦ 1; d is in the range of 0 ≦ d ≦ 1; e is in the range of 0 ≦ e ≦ 1;and n represents the number determined by a valence of component elements, said method comprising: a step of extracting a part of the catalyst from the reactor in a non-steady state of the ammoxidation reaction, wherein a steady-state refers to a state in which, when no change has been made to the temperature of the catalyst layer in the reactor or the composition of the raw material gas supplied to the reactor for 1 or more days, temperature fluctuations of the catalyst layer in the reactor over 1 day are no greater than 10 °C; a step of measuring at least one catalyst physical property value selected from the group consisting of the normalized UV value of the catalyst and the reduction ratio of the catalyst contained in a reactor; and a step of maintaining or changing a reaction condition based on the measured physical property value so that fluctuation in the physical property value of the catalyst is maintained within ± 30% of the physical property value of the catalyst prior to packing into the reactor, wherein in the step of maintaining or changing the reaction condition comprises performing at least one selected from the group consisting of maintaining or changing of the temperature of the catalyst layer in the reactor, and maintaining or changing the composition of raw material gases supplied to the reactor, and maintaining or changing a contact time between the catalyst and the raw material gases, and wherein the normalized UV value acts as an index of the catalyst redox state which is determined based on the absorbance at 400 nm, 580 nm, and 700 nm of the absorption and/or reflection spectrum obtained by measuring by a diffuse reflection method using a visible-ultraviolet spectrophotometer and calculated based on the following equation: Normalized UV value = 580 nm absorbance − 400 nm absorbance / 700 nm absorbance − 400 nm absorbance and wherein the reduction ratio of the catalyst is represented by the following equation: Reduction ratio % = ( n 0 − n / n 0 × 100 wherein n denotes the number of oxygen atoms satisfying the valence of the constituent elements other than oxygen in the catalyst; and no denotes the number of oxygen atoms required when the constituent elements other than oxygen in the catalyst each have their maximum oxidation number. a step of extracting a part of the catalyst from the reactor in a non-steady state of the ammoxidation reaction, wherein a steady-state refers to a state in which, when no change has been made to the temperature of the catalyst layer in the reactor or the composition of the raw material gas supplied to the reactor for 1 or more days, temperature fluctuations of the catalyst layer in the reactor over 1 day are no greater than 10 °C; a step of measuring at least one catalyst physical property value selected from the group consisting of the normalized UV value of the catalyst and the reduction ratio of the catalyst contained in a reactor; and a step of maintaining or changing a reaction condition based on the measured physical property value so that fluctuation in the physical property value of the catalyst is maintained within ± 30% of the physical property value of the catalyst prior to packing into the reactor, wherein in the step of maintaining or changing the reaction condition comprises performing at least one selected from the group consisting of maintaining or changing of the temperature of the catalyst layer in the reactor, and maintaining or changing the composition of raw material gases supplied to the reactor, and maintaining or changing a contact time between the catalyst and the raw material gases, and wherein the normalized UV value acts as an index of the catalyst redox state which is determined based on the absorbance at 400 nm, 580 nm, and 700 nm of the absorption and/or reflection spectrum obtained by measuring by a diffuse reflection method using a visible-ultraviolet spectrophotometer and calculated based on the following equation: Normalized UV value = 580 nm absorbance − 400 nm absorbance / 700 nm absorbance − 400 nm absorbance and wherein the reduction ratio of the catalyst is represented by the following equation: Reduction ratio % = ( n 0 − n / n 0 × 100 wherein n denotes the number of oxygen atoms satisfying the valence of the constituent elements other than oxygen in the catalyst; and no denotes the number of oxygen atoms required when the constituent elements other than oxygen in the catalyst each have their maximum oxidation number.