plant the housing 32 is a huge cylinder whose diameter is 4 to 5 meters and the inner diameter of the housing 32 has a circumferential distortion deviation from a true circle of about ten odd mm then high roundness can be set in the inner diameter of the outside slope part 43 b for example by adjustment of the attachment position of the outside slope part 43 b or machining of the outside slope part 43 b thus the gap 45 between the inside slope part 43 a and the outside slope part 43 b can be adjusted to be about several mm easily 2 a part of the particles 63 which are falling down can be blown to above the pulverizing table 2 by the primary air 61 jetted upward from the gap 45 it has been confirmed by flow analysis or the like that in spite of the gap 45 about several mm wide the flow velocity of the primary air 61 jetted therefrom becomes substantially equal to the flow velocity of the primary air 61 flowing in the annular flow channel and the flow velocity reaches several tens m s in this manner a part of the particles 68 sliding down on the slope part 43 b are blown away so that the amount of the particles 68 arriving at the top end surface 40 a.