A process for producing acetic acid, which comprises: (a) a reaction step for continuously allowing methanol to react with carbon monoxide in the presence of a catalyst system comprising a metal catalyst, a halide salt, and methyl iodide in a carbonylation reactor, (b) a flash evaporation step for continuously feeding a flasher with a reaction mixture from the reactor and separating a lower boiling point component containing product acetic acid and methyl iodide and a higher boiling point component containing the metal catalyst and the halide salt, (c) an acetic acid collection step for continuously feeding a distillation column with the lower boiling point component, and separating a lower boiling point component containing methyl iodide and by-product acetaldehyde and a stream containing acetic acid to collect acetic acid, (d) a condensation step for condensing and temporarily holding the lower boiling point component in a decanter and discharging the lower boiling point component from the decanter, and (e) a separation and recycling step for separating the lower boiling point component discharged from the decanter into acetaldehyde and a liquid residue and recycling the liquid residue to a step from the reaction step to the acetaldehyde-separation step, wherein in the separation and recycling step, the lower boiling point component is fed to an acetaldehyde separation column and separated into a lower boiling point component containing acetaldehyde and a higher boiling point component, as the liquid residue for recycling, containing methyl iodide by distillation, wherein the process is a continuous process in which, in the acetic acid collection step (c), the flow rate of the lower boiling point component to be fed to the distillation column is 90 to 110, assuming that the average flow rate of the lower boiling point component to be fed to the distillation column is 100; and in the condensation step (d), the flow rate of the lower boiling point component to be fed to the decanter is 80 to 120, assuming that the average flow rate of the lower boiling point component to be fed to the decanter is 100; wherein in the condensation step (d), the amount of the lower boiling point component to be held is adjusted or controlled based on a fluctuating flow rate of the lower boiling point component to be fed to the decanter, and the amount of the lower boiling point component to be fed to the separation and recycling step (e) is adjusted or controlled by recycling part of the lower boiling point component discharged from the decanter to the carbonylation reactor and/or to the distillation column, so that, assuming that the average flow rate of the lower boiling point component to be fed to the separation and recycling step is 100, the flow rate of the lower boiling point component to be fed to the separation and recycling step is adjusted so as to be 90 to 110. (a) a reaction step for continuously allowing methanol to react with carbon monoxide in the presence of a catalyst system comprising a metal catalyst, a halide salt, and methyl iodide in a carbonylation reactor, (b) a flash evaporation step for continuously feeding a flasher with a reaction mixture from the reactor and separating a lower boiling point component containing product acetic acid and methyl iodide and a higher boiling point component containing the metal catalyst and the halide salt, (c) an acetic acid collection step for continuously feeding a distillation column with the lower boiling point component, and separating a lower boiling point component containing methyl iodide and by-product acetaldehyde and a stream containing acetic acid to collect acetic acid, (d) a condensation step for condensing and temporarily holding the lower boiling point component in a decanter and discharging the lower boiling point component from the decanter, and (e) a separation and recycling step for separating the lower boiling point component discharged from the decanter into acetaldehyde and a liquid residue and recycling the liquid residue to a step from the reaction step to the acetaldehyde-separation step, wherein in the separation and recycling step, the lower boiling point component is fed to an acetaldehyde separation column and separated into a lower boiling point component containing acetaldehyde and a higher boiling point component, as the liquid residue for recycling, containing methyl iodide by distillation,