A saccharified solution production method includes: a saccharifying step of saccharifying hemicellulose or cellulose contained in cellulosic biomass to C5 or C6 saccharides by subjecting a slurry of cellulosic biomass to a hot water treatment in a supercritical state or subcritical state; a washing step of successively washing a solid in the slurry with washing water after the saccharifying step, by using multiple stages of thickeners for washing arranged in series so that the direction of movement of the solid in the slurry and the direction of movement of overflow water are opposite to each other; and a concentration step of removing a solid residue from the washing water recovered in the washing step by using a thickener for still standing that is different from the thickeners for washing, and then concentrating a supernatant of the thickener for still standing by using a concentration device to give a saccharified solution.