A process for producing acetic acid by: a reaction step for continuously allowing methanol to react with carbon monoxide in the presence of a catalyst system comprising a metal catalyst, an ionic iodide, and methyl iodide in a carbonylation reactor, a flash distillation step for continuously feeding a flasher with a reaction mixture from the reactor and evaporating a volatile component at least containing product acetic acid, methyl acetate, and methyl iodide by flash distillation to separate the volatile component and a liquid catalyst mixture at least containing the metal catalyst and the ionic iodide, and an acetic acid collection step for separating a stream containing acetic acid from the volatile component to collect acetic acid; wherein, in the flash distillation step, the flash distillation is conducted under the condition that the concentration of methyl acetate in the liquid catalyst mixture is not less than 0.6% by weight.