An apparatus comprises: a hopper for feeding biomass; a horizontal extruder comprising a cylindrical housing and a rotatable screw mounted in the housing, in which one side of the housing is connected to the hopper, and an outlet provided at the other side of the housing is connected to an inlet for feeding biomass into a refiner; and a refiner comprising a stator disc having a cooling water space formed on the outer peripheral surface thereof, one or more enzyme feeding holes provided in a refining region adjacent to the circumference of the stator disc, and a biomass inlet provided at the central portion thereof, and a refining disc which is disposed so as to form a gap with the inner surface of the stator disc and to have a refining region opposite to that of the stator disc and is rotated by a separate motor.