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 said cellulosic biomass into a water-permeable vessel and then encapsulating said water-permeable vessel and water into each of said pressure vessels; said heating-up step is a step of hermetically closing the pressure vessel and heating up said cellulosic biomass and water; 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 high-temperature and high-pressure water contained in the pressure vessel to lower the temperature thereof; said discharging step is a step of removing said water and said water-permeable vessel out of said 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); and 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 water to be charged into the pressure vessel performing said charging step and high-temperature water 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 said temperature lowering step to the pressure vessel performing said heating-up step. said charging step is a step of charging said cellulosic biomass into a water-permeable vessel and then encapsulating said water-permeable vessel and water into each of said pressure vessels; said heating-up step is a step of hermetically closing the pressure vessel and heating up said cellulosic biomass and water; 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 high-temperature and high-pressure water contained in the pressure vessel to lower the temperature thereof; said discharging step is a step of removing said water and said water-permeable vessel out of said 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); and 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 water to be charged into the pressure vessel performing said charging step and high-temperature water 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 said temperature lowering step to the pressure vessel performing said heating-up step.