A method of producing glycogen comprising a step of: allowing a branching enzyme having the ability to synthesize glycogen to act on a substrate in a solution to produce a glycogen, wherein the substrate is an .alpha.-glucan being linked mainly with .alpha.-1,4-glucosidic bonds and having a degree of polymerization of 4 or more, and the number-average molecular weight of saccharides in the solution before initiation of the reaction is more than 180 but not more than 150,000. (The branching enzyme activity of the branching enzyme)/(the molecular-weight-decreasing activity of the branching enzyme) can be 500 or less. The branching enzyme can be a thermostable branching enzyme.