A method for producing a concentrated saccharified solution using lignocellulosic biomass as a source material. The method includes filtering a saccharified solution obtained by hydrolysis of cellulosic biomass through a membrane separator having a separation membrane having a molecular cutoff ranging from 1000 or more and 7000 or less, and feeding it to a reverse osmosis membrane device to concentrate the same. By such treatment, the saccharides having small molecular weights contained in the saccharified solution are transferred to the filtrate, and the lignin and its decomposition product having large molecular weights are drained as a concentrated solution from the membrane separator. Accordingly, it is possible to prevent loss of saccharides contained in the saccharified solution, and to easily prevent occurrence of fouling in the reverse osmosis membrane used in concentration of the saccharified solution.