A process for producing a cytosine derivative which comprises:(i) producing a 4-O-sulfonyluracil derivative having the formula: ##STR6## wherein R1 is a hydrogen atom, a halogen atom, an alkyl group or an alkoxyl group, R2 is a glycosyl group whose hydroxyl groups are protected, and R3 SO2 is an organic sulfonyl group by reacting a uracil derivative having the formula: ##STR7## wherein R1 and R2 a