A method for decomposing cellulose to be contained in a cellulose raw material is provided. A pulverized cellulose based biomass is enclosed in a pressure closed vessel, and a sodium hydroxide aqueous solution having a concentration of 5%, pure water and 5 g of nickel oxyhydroxide obtained by solid-solving therein at least one kind of zinc, aluminum, magnesium, calcium, manganese, cobalt, copper and tin relative to nickel are added to prepare a catalytic reaction solution. Next, the catalytic reaction solution for decomposing cellulose by using nickel oxyhydroxide as a catalyst is subjected to a decomposition reaction of cellulose while stirring by using a stirring blade and heating at a temperature rising rate of 5.degree. C./min. The reaction is carried out under autogenous pressure (saturated vapor pressure of water) in the reactor. After the temperature of the catalytic reaction solution has reached a prescribed temperature, the resulting catalytic reaction solution is heated for one hour and then cooled to room temperature at a rate of about 3.degree. C./min.