A process for preparing a thiazole compound of formula (3): wherein X.sup.1 represents a hydrogen atom or a halogen atom comprising reacting a compound of formula (1): wherein X.sup.1 is as defined above, and X.sup.2 represents a halogen atom, with ammonia and formaldehyde to obtain a hexahydrotriazine compound of formula (2): wherein X.sup.1 is as defined above, and hydrolyzing the compound of formula (2). ##STR00001##