A method of hydrolytic saccharification of a cellulosic biomass with use of plural pressure vessels, the method comprising a discharging and charging step, a heating-up step, a hydrolyzing step, and a temperature lowering step, which are performed sequentially by each of said pressure vessels, wherein: said discharging and charging step is a step of removing a slurry out of each of said pressure vessels after said temperature lowering step and charging a slurry prepared by grinding said cellulosic biomass and mixing said cellulosic biomass thus ground with water into the same pressure vessel; said heating-up step is a step of hermetically closing the pressure vessel and heating up the pressure vessel; 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; equal time is required to complete respective of all the three 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 three steps; and the number of the pressure vessels used is a multiple of (3+n); 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 discharging and charging step is a step of removing a slurry out of each of said pressure vessels after said temperature lowering step and charging a slurry prepared by grinding said cellulosic biomass and mixing said cellulosic biomass thus ground with water into the same pressure vessel; said heating-up step is a step of hermetically closing the pressure vessel and heating up the pressure vessel; 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; equal time is required to complete respective of all the three 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 three steps; and the number of the pressure vessels used is a multiple of (3+n); 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.