A method for producing acetic acid by reacting methanol with carbon monoxide in the presence of a catalyst system comprising a metal catalyst and methyl iodide, acetic acid, methyl acetate, and water, comprising: a separation step of separating a process stream containing at least water, acetic acid, methyl iodide, and acetaldehyde into an aqueous phase and an organic phase; and an acetaldehyde separation and removal step for separating and removing the acetaldehyde derived from the process stream, wherein the separation step satisfies the following condition (i), the following condition (iii), at least one of the following conditions (iv) and (v), and optionally the following condition (ii), and at least a portion of the organic phase is treated in the acetaldehyde separation and removal step: (i) an acetaldehyde concentration in the aqueous phase is not less than 0.045% by mass, and/or an acetaldehyde concentration in the organic phase is not less than 0.013% by mass; (ii) a temperature at the time of the separation is not less than -5°C; (iii) a methyl acetate concentration in the organic phase is not less than 5.8% and not more than 60% by mass; (iv) an acetaldehyde distribution coefficient [{acetaldehyde concentration (% by mass) of the aqueous phase} / {acetaldehyde concentration (% by mass) of the organic phase}] is not more than 4.1; and (v) a methyl acetate distribution coefficient [{methyl acetate concentration (% by mass) of the aqueous phase} / {methyl acetate concentration (% by mass) of the organic phase}] is not more than 0.53. a separation step of separating a process stream containing at least water, acetic acid, methyl iodide, and acetaldehyde into an aqueous phase and an organic phase; and an acetaldehyde separation and removal step for separating and removing the acetaldehyde derived from the process stream, wherein the separation step satisfies the following condition (i), the following condition (iii), at least one of the following conditions (iv) and (v), and optionally the following condition (ii), and at least a portion of the organic phase is treated in the acetaldehyde separation and removal step: (i) an acetaldehyde concentration in the aqueous phase is not less than 0.045% by mass, and/or an acetaldehyde concentration in the organic phase is not less than 0.013% by mass; (ii) a temperature at the time of the separation is not less than -5°C; (iii) a methyl acetate concentration in the organic phase is not less than 5.8% and not more than 60% by mass; (iv) an acetaldehyde distribution coefficient [{acetaldehyde concentration (% by mass) of the aqueous phase} / {acetaldehyde concentration (% by mass) of the organic phase}] is not more than 4.1; and (v) a methyl acetate distribution coefficient [{methyl acetate concentration (% by mass) of the aqueous phase} / {methyl acetate concentration (% by mass) of the organic phase}] is not more than 0.53. (i) an acetaldehyde concentration in the aqueous phase is not less than 0.045% by mass, and/or an acetaldehyde concentration in the organic phase is not less than 0.013% by mass; (ii) a temperature at the time of the separation is not less than -5°C; (iii) a methyl acetate concentration in the organic phase is not less than 5.8% and not more than 60% by mass; (iv) an acetaldehyde distribution coefficient [{acetaldehyde concentration (% by mass) of the aqueous phase} / {acetaldehyde concentration (% by mass) of the organic phase}] is not more than 4.1; and (v) a methyl acetate distribution coefficient [{methyl acetate concentration (% by mass) of the aqueous phase} / {methyl acetate concentration (% by mass) of the organic phase}] is not more than 0.53.