After the object of the present invention is a method for producing ethanol in a cellulosic biomass as a feedstock, the use of volatile organic acids decomposing hemicellulose saccharification as a catalyst, the recovery of volatile organic acids from C5 saccharified liquid and a half saccharification of cellulose decomposing catalyst is reused, thereby suppressing the processing costs. The present invention is a volatile organic acid as a catalyst is added to the slurry and saccharification of hemicellulose decomposition reaction, a small amount of sulfuric acid was added to the saccharified solution C5, and C5 saccharified solution to air stripping process, this can be easily C5 saccharified solution to recovery of volatile organic acid containing (volatile organic acids and by-produced volatile organic acid is added in the slurry). When the recovered volatile organic acids further added to the cellulosic biomass slurry as a catalyst can be continuously improve the saccharification efficiency decomposition hemicellulose saccharification by decomposition of volatile organic acids C5 sugars .