Abstract: The present invention provides a process capable of industrially producing 5-deoxy-L-arabinose, important as a raw material for the production of sapropterin useful as a therapeutic agent for atypical hyperphenylalaninemia, satisfactorily efficiently even with a simple production apparatus. Provided is the process for producing 5-deoxy-L-arabinose characterized by including: reacting L-rhamnose with a C11-16 straight chain alkyl mercaptan compound in the presence of an acid catalyst to prepare L-rhamnose dialkylmercaptal; subjecting then the obtained compound to an oxidation reaction to prepare a sulfonyl derivative; and subjecting then the sulfonyl derivative to a carbon-reduction reaction to prepare 5-deoxy-L-arabinose.