Abstract: The present invention provides a method of preparing stably and at a high yield a purine nucleoside compound by utilizing an exchange reaction of a nucleic acid base which is carried out in the presence of an enzyme, and also provides a microorganism capable of producing uracil thymine dehydrogenase or dihydrouracil dehydrogenase. In preparing the purine nucleoside compound, a pyrimidine nucleoside compound and a purine base are subjected to a base exchange reaction in an aqueous solution containing phosphate ions in the presence of pyrimidine nucleoside phosphorylase and purine nucleoside phosphorylase. The pyrimidine base formed by the base exchange reaction is converted by a microorganism or an enzyme derived from the microorganism into a compound incapable of acting as substrates of pyrimidine nucleoside phosphorylase and purine nucleoside phosphorylase so as to obtain a desired product of purine nucleoside compound.