A desired isomer is selectively prepared by phosphorolyzing and isomerizing an anomer mixture of a 1-phosphorylated saccharide derivative while crystallizing one of the isomers to displace the equilibrium. Furthermore, using the action of a nucleoside phosphorylase, a nucleoside is prepared from the 1-phosphorylated saccharide derivative obtained and a base with improved stereoselectivity and a higher yield. This process is an anomer-selective process for preparing a 1-phosphorylated saccharide derivative and a nucleoside.