To produce highly pure 1,3-thiazolidin-2-one by using an easily available inexpensive raw material by a simple method by reacting ethanol amine with hydrogen chloride, and further reacting the product with carbonyl sulfide without purifying the product.

(i) Ethanolamine is reacted with hydrogen chloride to provide 2-chloroethylamine hydrochloride, and (ii) water is added to the obtained 2- chloroethylamine hydrochloride without purifying the 2-chloroethylamine hydrochloride. The obtained mixture is subjected to filtration to remove a solid component, and the filtrate is reacted with carbonyl sulfide in the presence of a base (e.g. sodium hydroxide) to provide the objective 1,3-thiazolidin-2-one. The reaction of (i) is preferably carried out in the presence of a solid acid catalyst (e.g. zeolite). Preferably, the hydrogen chloride of 1 eq. is blown into the ethanolamine at 90-110°C, and further 1-6 eq. thereof is blown thereinto after the temperature is raised to 140-180°C. The reaction of (ii) is preferably carried out by using a secondary alcohol (e.g. 2-propanol) as a solvent. After finishing the reaction, the objective product is extracted by adding 1-butanol or the like.