A process of producing cellooligosaccharide, comprising enzymatically decomposing, in the presence of cellulase, a water-insoluble natural cellulosic material having an average degree of polymerization not greater than 700, an average particle size not greater than 100. A process of producing cellooligosaccharide, comprising enzymatically decomposing, in the presence of cellulase, a water-insoluble natural cellulosic material having an average degree of polymerization not greater than 700, containing 10% or more by mass of a colloidal cellulose component and a diethyl ether-soluble substance content less than 1% by mass.