The apparatus includes: a pretreatment tank where biomass and a first acid solution are stirred to extract sugar components from the biomass; a hydrolysis tank where water is added to the pretreated mixture transferred from the pretreatment tank such that the concentration of the acid is reduced and the sugar components are hydrolyzed to produce an acid hydrolyzate; a first sugar-acid separation tank where the acid hydrolyzate is separated into a second acid solution and a first hydrolyzate; a second sugar-acid separation tank where the first hydrolyzate is separated into a third acid solution and a second hydrolyzate; a fermentation tank where the second hydrolyzate is fermented to produce bioenergy; and an acid solution concentration tank where a mixture of the second acid solution transferred from the first sugar-acid separation tank and the third acid solution transferred from the second sugar-acid separation tank is concentrated to a higher level for reuse.