A classification device for classifying particles by size including an upper impeller-type rotor, a first end of which is covered with a cover disc without penetration hole and a second end of which is covered with an upper cover disc with a penetration hole; and a lower impeller-type rotor, a first end of which is covered with a cover disc without penetration hole and a second end of which is covered with a lower cover disc with a penetration hole. The upper impeller-type rotor and the lower impeller-type rotor are provided so that axes thereof are coincident and the first ends face with each other forming a gap therebetween in an axial direction, and the diameters of the penetration holes of the upper cover disc and the lower cover disc are different.