A method of hydrolytic saccharification of a cellulosic biomass with use of plural pressure vessels, the method comprising a charging step, a heating-up step, a hydrolyzing step, a temperature lowering step, and a discharging step, which are performed sequentially by each of said pressure vessels, wherein: said charging step is a step of charging a slurry prepared by grinding said cellulosic biomass and then mixing said cellulosic biomass thus ground with water into each of said pressure vessels; said heating-up step is a step of hermetically closing the pressure vessel and heating up said slurry; said hydrolyzing step is a step of hydrolyzing cellulose and/or hemicellulose contained in said cellulosic biomass into saccharides by an oxidative power of high-temperature and high-pressure water; said temperature lowering step is a step of flash-evaporating the high-temperature and high-pressure slurry contained in the pressure vessel to lower the temperature thereof; said discharging step is a step of removing the slurry out of the pressure vessel; equal time is required to complete respective of all the four steps other than said hydrolyzing step; the time required to complete said hydrolyzing step is n times (where n is a natural number) as long as the time required to complete each of the other four steps; and the number of the pressure vessels used is a multiple of (4+n); while any one of said plural pressure vessels performs said charging step, any one of the other pressure vessels performs said discharging step so as to allow heat exchange to occur between the slurry to be charged into the pressure vessel performing the charging step and the slurry to be discharged from the pressure vessel performing said discharging step; and while any one of said plural pressure vessels performs said heating-up step, any one of the other pressure vessels performs said temperature lowering step and allows heat recovery to be made by supplying flash vapor discharged from the pressure vessel performing the temperature lowering step to the pressure vessel performing the heating-up step. said charging step is a step of charging a slurry prepared by grinding said cellulosic biomass and then mixing said cellulosic biomass thus ground with water into each of said pressure vessels; said heating-up step is a step of hermetically closing the pressure vessel and heating up said slurry; said hydrolyzing step is a step of hydrolyzing cellulose and/or hemicellulose contained in said cellulosic biomass into saccharides by an oxidative power of high-temperature and high-pressure water; said temperature lowering step is a step of flash-evaporating the high-temperature and high-pressure slurry contained in the pressure vessel to lower the temperature thereof; said discharging step is a step of removing the slurry out of the pressure vessel; equal time is required to complete respective of all the four steps other than said hydrolyzing step; the time required to complete said hydrolyzing step is n times (where n is a natural number) as long as the time required to complete each of the other four steps; and the number of the pressure vessels used is a multiple of (4+n); while any one of said plural pressure vessels performs said charging step, any one of the other pressure vessels performs said discharging step so as to allow heat exchange to occur between the slurry to be charged into the pressure vessel performing the charging step and the slurry to be discharged from the pressure vessel performing said discharging step; and while any one of said plural pressure vessels performs said heating-up step, any one of the other pressure vessels performs said temperature lowering step and allows heat recovery to be made by supplying flash vapor discharged from the pressure vessel performing the temperature lowering step to the pressure vessel performing the heating-up step.