To obtain the subject compound having sufficient quality for industrial uses by carrying out operation of carbonylation by using a specified liquid composition such as CH3OH in the presence of an Ir catalyst.

CH3OH, etc., and CO are charged into a reactor and a liquid composition comprising Ir carbonylation catalyst, CH3I cocatalyst, water at about <8 wt.% concentration, methyl acetate, etc., is maintained in the reactor during process of the treatment and then, the liquid composition is recovered from the reactor and a liquid fraction comprising water, acetic acid product, unvolatile Ir catalyst, a promoter, etc., is formed and then, the liquid fraction is recycled from a flash zone to the reactor and introduced to a first distillation zone. Light ends recycle stream comprising water, etc., is taken out from a point above the introduction point of flash area and recycled. Acetic acid product and a process stream containing <1,500 ppm water are taken out from a point below the introduction point in the first distillation zone. When the process stream contains ≥400 ppm water, the acetic acid product and the process stream are introduced into a second distillation column and when the process stream contains <400 ppm propionic acid and 1,500 ppm water, the stream is taken out from the upper part of the second distillation column to provide the objective acetic acid.