In reacting pentose-1-phosphoric acid with a nucleic acid base or a nucleic acid base analogue in an aqueous reaction medium in the presence-of a metal cation to produce a nucleoside compound, the timing or method of addition of at least one of these components to the aqueous reaction medium is varied; thereby, a nucleoside compound can be produced at a high yield efficiently without inviting the high viscosity or solidification of the reaction mixture, even when the above components are used in such amounts that the reaction mixture becomes highly viscous or is solidified when the components are used without the above variation of the addition timing or method. Thus, there can be provided a process for producing a nucleoside compound, which comprises a step of reacting pentose-1-phosphoric acid with a nucleic acid base or a nucleic acid base analogue in the presence of nucleoside phosphorylase activity, which gives a nucleoside compound at an improved conversion, and which has wide applicability.