In a process for producing a nucleoside compound by reacting pentose-1-phosphate with a nucleic acid base or a nucleic acid base analog in an aqueous reaction medium in the presence of a metal cation, the point and method of adding at least one of these reactants to the aqueous reaction medium are changed to thereby effectively produce the nucleotide compound at a high yield without causing an increase in viscosity of the liquid reaction mixture or the solidification thereof even in case of using the reactant at such an amount as causing the increase in viscosity of the liquid reaction mixture or the solidification thereof when employed as such. Thus, it becomes possible to provide a widely applicable process for producing a nucleoside compound which involves the step of reacting pentose-1-phosphate with a nucleic acid base or a nucleic acid base analog in the presence of a nucleoside phosphorylase activity and by which the conversion ratio of the nucleoside compound in the above reaction can be elevated.