To provide a method for efficiently separating and removing acetaldehyde from a liquid mixture of acetaldehyde, methyl iodide and water or a method for removing easily and sufficiently acetaldehyde contained in a process liquid to be recycled to a carbonylation reactor and recycling efficiently methyl iodide and water to the reactor in the production of acetic acid by carbonylation of methanol.

In the method, acetaldehyde is efficiently removed by distilling a liquid mixture containing acetaldehyde of not higher than 5% by weight, methyl acetate of not higher than 10% by weight, acetic acid of 20 to 50% by weight, methyl iodide of 0.1 to 20% by weight and water of 5 to 50% by weight with a distillation column having not less than 40 stages at a reflux ratio of not lower than 10.