To provide a method for producing L-biopterin on a large industrial scale by using a reagent which is inexpensive and easy to handle, without requiring a use of any particular equipment or plants.A method for porducing a biopterin derivative represented by the formula (6): ##STR00001## wherein R.sup.1 and R.sup.2, which are the same or different from each other, each represents a hydrogen atom, an alkyl group, or an aryl group, comprising: reacting a compound belonging to triacetoxy-5-deoxy-L-arabinose phenylhydrazones represented by the formula (4): ##STR00002## wherein R.sup.1 and R.sup.2 are the same as defined above, with 6-hydroxy-2,4,5-triaminopyrimidine (5) under catalytic influence of a Lewis acid in an aqueous solvent.