A method for producing P1,P4-di(uridine 5'-)tetraphosphate (UP4U) is developed, which does not require the use of free UTP and does not undergo the decrease in synthesis efficiency. A method for producing UP4U, characterized by reacting a phosphoric-acid-activating compound represented by formula [II] or [III] with a phosphoric acid compound selected from the group consisting of UMP, UDP, UTP and pyrophosphoric acid or a salt thereof (excluding free UTP) in water or a hydrophilic organic solvent in the presence of a metal ion selected from the group consisting of an iron (II) ion, an iron (III) ion, a trivalent aluminum ion, a trivalent lanthanum ion and a trivalent cerium ion. (Each of R1, X and n in formula [II] and X in formula [III] is as defined in claim 1.)
