A process for producing methionine, while corrosion of a pipe and a reaction vessel is well inhibited, is provided including the following steps (1) to (3), wherein a content of thiols in 3 -methylthiopropanal is 500 ppm or less, based on the propanal, and a content of hydrogen sulfide in 3-methylthiopropanal is 60 ppm or less, based on the propanal; step (1) in which 3 -methylthiopropanal is reacted with hydrogen cyanide in the presence of a base to give 2 -hydroxy -4-methylthiobutanenitrile; step (2) in which the 2-hydroxy-4-methylthiobutanenitrile obtained in step (1) is reacted with ammonium carbonate to give 5-(β-methylmercaptoethyl) hydantoin; and step (3) in which the 5-(β-methylmercaptoethyl)hydantoin obtained in step (2) is hydrolyzed in the presence of a basic potassium compound to give methionine.