The present invention provides a method of manufacturing a cellooligosaccharide containing an oligomer having a degree of polymerization of from 3 to 6, the method comprising performing a reaction involving heating and hydrolyzing plant biomass in coexistence with a carbon catalyst and water under such a condition that a temperature-time product over a range of from 170°C to 230°C in a graph for showing a relationship between a reaction temperature on an axis of ordinate and a reaction time on an axis of abscissa is from 80°C·min to 365°C·min. The maximum reaction temperature (X) and the retention time (Y) at the maximum reaction temperature preferably satisfy a relationship represented by the following expressions (1) to (3). Y > ˆ’ 0.0714 X + 15 Y < ˆ’ X + 210 X > 175
