A method of producing glycogen is provided. The method comprises the step of heat- and pressure-treating a sugar-containing material under acidic conditions. The sugar-containing material is a polysaccharide or an oligosaccharide. Alternatively, the sugar-containing material is a plant material selected from the group consisting of Panax notoginseng, Yun Nan San-chi powder (trademark), Panax ginseng, wheat flour, soybean, soy flour, shiitake, and coffee extract residue. Representatively, the glycogen includes a molecule having a molecular weight of 10,000 or less. The glycogen has a specific rotation of [.alpha.].sub.D+197.degree. and anomeric proton peaks at 5.37 ppm and 4.95 to 5.33 ppm in .sup.1H NMR spectra.