The present invention relates to a method of manufacturing cellulose acetate by use of a high temperature steam reactor vessel and a superheated steam generator. According to this method, a material to which solid catalyst added is subject to a steaming process so as to cause cellulose content to be separated from the material, followed by carrying out an acetylating process of the separated cellulose content together with solid acid in the state of being pressurized so as to obtain cellulose acetate. The high temperature steam reactor vessel employs a system that there is arranged in a reactor vessel body 1 a cartridge 2 filled with collective chips 100, whereby enabling the material subjected to the process to be readily handled and processed, and also resulting in that there is no need to carry out in a later process separation between useful content and residue produced in hydrolysis with high temperature steam. The superheated steam generator comprises a heat-exchange line 3 having line parts P1, P2, P3 sectioned into plural stages and so adapted that a sectional area of the line part on a stage at a downstream side is larger than that of the line part on a stage at an upstream side, whereby superheated steam having ultra high temperature is able to be generated while or although pressure proof properties can be designed to be relatively lower.