A method for producing nucleoside-5′-phosphate esters inexpensively and in high yields by phosphorylating a nucleoside with a phospahte group donor using an acid phosphatase having an increased affinity for the nucleoside and/or an increased temperature stability at a pH of pH 3.0 to 5.5, to produce a nucleoside-5′-phosphate ester. Mutant acid phosphatases having increased affinity for nucleosides and/or an enhanced temperature stability are also provided.