In the present disclosure, after a volatile organic acid is added as a catalyst to a slurry and the hydrolytic saccharification reaction of hemicellulose is performed, the volatile organic acid (including one added to the slurry and one produced as a by-product) contained in a C5 saccharified solution can be easily recovered by adding a small amount of sulfuric acid to the C5 saccharified solution and subjecting the C5 saccharified solution to air stripping treatment. The efficiency of hydrolytic saccharification of hemicellulose into C5 sugars using a volatile organic acid can also be continuously improved by adding the recovered volatile organic acid as a catalyst to another slurry of cellulosic biomass.