The present invention aims to provide a method for producing a polysaccharide with high efficiency using a polysaccharide synthase. The present invention provides a method for producing a polysaccharide, including allowing polysaccharide synthase (B) to act on ribonucleoside diphosphate-monosaccharide (A) to produce a polysaccharide, wherein in 10 to 100% of the duration in which (B) acts on (A), the concentration of ribonucleoside diphosphate in a reaction solution is lower than 100 times an inhibitory concentration IC50 against polysaccharide synthase (B).