At a conveyor forefront portion, velocity distribution of an airflow is provided. The velocity distribution is wind velocity distribution in a vertical direction of an airflow from a surface of an upper rectifying plate to a surface of a conveyor at the conveyor forefront portion, and has a maximum value in a range of less than 10 mm downward of the vertical direction from the rectifying plate surface. Moreover, a ratio obtained by dividing the maximum value by a wind velocity in the vicinity of the surface of the conveyor is 4 or more, and 12 or less. In a range other than the range of less than 10 mm, the airflow has a wind velocity equal to the wind velocity in the vicinity of the surface of the conveyor.