A method of producing a saccharified solution, in which saccharified slurry is obtained by hydrothermally treating cellulosic biomass slurry in a supercritical state or a subcritical state by a continuous reactor. The method reduces the risk of a pressure reducing valve becoming clogged when the high-temperature and high-pressure slurry is flash-evaporated after being hydrothermally treated. Also provides an apparatus for producing a saccharified solution and a continuous reactor. At the time of hydrothermally treating the cellulosic biomass slurry by the continuous reactor continuously, the slurry is intensely stirred in a front part of the continuous reactor, and stirred in a rear part of the continuous reactor by a stirring device with low propelling force, such that the plug flow in the axial direction is kept while the slurry is being uniformly mixed with respect to the cross section perpendicular to the mixing axis.