An air-fuel ratio control method for post injection in an exhaust gas purification device that is configured to be able to execute the post injection for regeneration of a nitrogen oxide occlusion catalyst that occludes nitrogen oxides discharged by an internal combustion engine, the air-fuel ratio control method comprising the steps of: obtaining an average post-injection amount as a post injection amount that is unambiguously defined for each cylinder (41-1 to 41-n) of the internal combustion engine on the basis of PID control in which an air-fuel ratio detected by a lambda sensor and a target average air-fuel ratio as a target air-fuel ratio are input; computing a post-injection deviation by cylinder (41-1 to 41-n) that corresponds to an excess/shortage amount in the post injection amount for each of the cylinders (41-1 to 41-n) with respect to the average post-injection amount on the basis of a characteristic deviation by cylinder (41-1 to 41-n) that corresponds to a variation in the air-fuel ratio of each of the cylinders (41-1 to 41-n); and performing the post injection by setting an addition result obtained by adding the post-injection deviation by cylinder (41-1 to 41-n) to the average post-injection amount as a post-injection amount by cylinder (41-1 to 41-n) that corresponds to the post injection amount required in each of the cylinders (41-1 to 41-n) to realize the target average air-fuel ratio, and thereby enabling achievement of the target average air-fuel ratio; wherein an approximate actual exhaust temperature deviation is computed that is a difference between an average value of exhaust temperatures of the cylinders (41-1 to 41-n) and the exhaust temperature of the cylinder (41-1 to 41-n) for which the characteristic deviation by cylinder (41-1 to 41-n) is computed, and wherein the characteristic deviation by cylinder (41-1 to 41-n) is obtained based on the approximate actual exhaust temperature deviation; characterized in that a theoretical exhaust temperature is determined for each of the cylinders (41-1 to 41-n) as a standard exhaust temperature on the basis of an engine speed and a main injection amount as an injection amount in main injection in the internal combustion engine, a standard exhaust temperature deviation is computed that is a difference between an average value of standard exhaust temperatures and the standard exhaust temperature in the cylinder (41-1 to 41-n) for which the characteristic deviation by cylinder (41-1 to 41-n) is computed, and the characteristic deviation by cylinder (41-1 to 41-n) is obtained as a difference between the approximate actual exhaust temperature deviation and the standard exhaust temperature deviation. obtaining an average post-injection amount as a post injection amount that is unambiguously defined for each cylinder (41-1 to 41-n) of the internal combustion engine on the basis of PID control in which an air-fuel ratio detected by a lambda sensor and a target average air-fuel ratio as a target air-fuel ratio are input; computing a post-injection deviation by cylinder (41-1 to 41-n) that corresponds to an excess/shortage amount in the post injection amount for each of the cylinders (41-1 to 41-n) with respect to the average post-injection amount on the basis of a characteristic deviation by cylinder (41-1 to 41-n) that corresponds to a variation in the air-fuel ratio of each of the cylinders (41-1 to 41-n); and performing the post injection by setting an addition result obtained by adding the post-injection deviation by cylinder (41-1 to 41-n) to the average post-injection amount as a post-injection amount by cylinder (41-1 to 41-n) that corresponds to the post injection amount required in each of the cylinders (41-1 to 41-n) to realize the target average air-fuel ratio, and thereby enabling achievement of the target average air-fuel ratio;