PURPOSE: To prevent a substance to be treated from being contaminated by foreign matters, by a method wherein, a pair of horizontal side plates arranged distant each other, and a support base interposed out of contact with them, are laid out of contact with the inner wall of a treatment vessel, and either of the bide plate and the support plate is loopedly rotated within the surface defined by the direction along which the substance to be treated is conveyed and the vertical directions.

CONSTITUTION: A pair of horizontal side walls 3 arranged distant each other within a treatment vessel 1, and a support base 4 are laid out of contact with each other while being parallel mutually with respect to the conveying direction of the substance 2 to be treated, both of the side wall and the support base 4 being arranged out of contact with the inner wall of the treatment vessel 1. Further, either of a pair of side plates 3 and the support plate 4 is driven and loopedly rotated within the surface defined by the direction along which the substance to be treated is conveyed and the vertical directions. By this arrangement, the substance to be treated 2, side plates 3 or a support plate are inhibited from making contact with the inner wall of the vessel. Thereby, the generation of foreign matters caused by the contact with the inner wall is prevented, leading to the prevention of contamination of the substance to be treated due to the foreign matters.