A process for efficiently producing a nucleic acid derivative by condensing a specific nitrogen-containing heterocyclic compound such as a dinitrogen 6-membered heterocyclic compound or a trinitrogen 5-membered heterocyclic compound with a pentose using a less expensive and safe reagent. Namely, a process for producing a nucleic acid derivative by condensing a dinitrogen 6-membered heterocyclic compound or a trinitrogen 5-membered heterocyclic compound with a pentose in the presence of an iron halide.
