NOVELTY - Reactor comprises (a) a rotary shaft that is rotatable by separate power, (b) several stirring units fixed to the rotary shaft, and (c) a cylindrical saccharification unit in which the rotary shaft passes through the central portion of the saccharification unit and the several stirring units fixed to the rotary shaft are arranged at predetermined distances from one another, where several perforated plates arranged between the stirring units in the cylindrical saccharification unit and configured to define a space including each of the stirring units.    USE - The reactor or method is useful for performing continuous saccharification of high-viscosity biomass (all claimed) which is used in bioethanol production.    ADVANTAGE - The reactor enables to control temperature and pH of the saccharification reaction simply.    DETAILED DESCRIPTION - Reactor comprises (a) a rotary shaft that is rotatable by separate power, (b) several stirring units fixed to the rotary shaft, and (c) a cylindrical saccharification unit in which the rotary shaft passes through the central portion of the saccharification unit and the several stirring units fixed to the rotary shaft are arranged at predetermined distances from one another, where several perforated plates arranged between the stirring units in the cylindrical saccharification unit and configured to define a space including each of the stirring units, each of the perforated plates includes one or more flow channels formed to extend from the outer circumferential surface of the perforated plate to the inside of the holes of the perforated plate, and nozzles fluidically communicating with the flow channels and the inside of the holes and configured to feed an additive. An INDEPENDENT CLAIM is included for method for continuous saccharification of high-viscosity biomass using the reactor, which involves (a) feeding an additive to the biomass through a nozzle fluidically communicating with the inside of the hole of the perforated plate by rotating a stirring units above each of perforated plates in a saccharification unit to allow biomass to reach the perforated plate, and mixing the additive with the biomass by rotation of a stirring units arranged under the perforated plate while passing the biomass through the hole while, and (b) saccharifying the biomass by moving the biomass downward in the saccharification unit while repeating step (a) by the perforated plates and stirring units sequentially arranged in the saccharification unit, where the additive that is fed through the nozzle are chosen from an enzyme, an acidic or basic substance for pH adjustment and a surfactant, which are fed through different nozzles.