A method of manufacturing a cellooligosaccharide containing an oligomer having a degree of polymerization of from 3 to 6, the method includes 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 satisfy a relationship represented by the following expressions (1) to (3).;Y>−0.0714X+15 (1);Y<−X+210 (2);X>175 (3)
