A classifier, comprising: a rotatable fin for executing a classification of solid particles on the basis of a centrifugal force; a tubular downward flow forming member provided in an outer peripheral side of the rotatable fin; and a bowl-shaped recovery cone arranged in a lower side of said rotatable fin and the downward flow forming member; a housing accommodating said rotatable fin, the downward flow forming member and the recovery cone, in which a contraction flow region is formed between the housing and the recovery cone for guiding a two-phase flow constituted by a mixture of said solid particles blown up through the contraction flow region from the lower side of the recovery cone and a gas, such that the particles in said two-phase flow are separated into fine particles and coarse particles by bringing the two-phase flow into collision with said downward flow forming member in an upper portion of said housing so as to form a downward flow, and the housing being configured for thereafter conducting the downward flow to said rotatable fin side, and for taking out the fine particles while passing through the portion between the rotatable fin rotating together with the air flow, wherein a circulating swirl flow development suppressing portion for suppressing a development of a circular swirl flow generated at its position is provided in an upper side of said contraction flow region and at an outer peripheral position of said downward flow forming member in such a manner as to have a lower end portion in a side wall upper portion of said housing and have an upper end portion in an outer peripheral portion of an upper surface plate, wherein said circulating swirl flow development suppressing portion is formed by a slant member provided in a joint portion between an upper portion between the upper end portion of a side wall of the housing and an outer peripheral portion of the upper surface plate so as to bridge said joint portion, wherein an angle of gradient of said circulating swirl flow development suppressing portion is regulated in a range between 15 and 75 degrees, wherein in the case that a distance from a side wall of said housing to said downward flow forming member is set to L, and a horizontal width from the side wall of the housing to an upper and end portion of said circulating swirl flow development suppressing portion is set to W, a ratio W/L is regulated in a range between 0.15 and 1.0. a rotatable fin for executing a classification of solid particles on the basis of a centrifugal force; a tubular downward flow forming member provided in an outer peripheral side of the rotatable fin; and a bowl-shaped recovery cone arranged in a lower side of said rotatable fin and the downward flow forming member; a housing accommodating said rotatable fin, the downward flow forming member and the recovery cone, in which a contraction flow region is formed between the housing and the recovery cone for guiding a two-phase flow constituted by a mixture of said solid particles blown up through the contraction flow region from the lower side of the recovery cone and a gas, such that the particles in said two-phase flow are separated into fine particles and coarse particles by bringing the two-phase flow into collision with said downward flow forming member in an upper portion of said housing so as to form a downward flow, and the housing being configured for thereafter conducting the downward flow to said rotatable fin side, and for taking out the fine particles while passing through the portion between the rotatable fin rotating together with the air flow, wherein a circulating swirl flow development suppressing portion for suppressing a development of a circular swirl flow generated at its position is provided in an upper side of said contraction flow region and at an outer peripheral position of said downward flow forming member in such a manner as to have a lower end portion in a side wall upper portion of said housing and have an upper end portion in an outer peripheral portion of an upper surface plate, wherein said circulating swirl flow development suppressing portion is formed by a slant member provided in a joint portion between an upper portion between the upper end portion of a side wall of the housing and an outer peripheral portion of the upper surface plate so as to bridge said joint portion, wherein an angle of gradient of said circulating swirl flow development suppressing portion is regulated in a range between 15 and 75 degrees, wherein in the case that a distance from a side wall of said housing to said downward flow forming member is set to L, and a horizontal width from the side wall of the housing to an upper and end portion of said circulating swirl flow development suppressing portion is set to W, a ratio W/L is regulated in a range between 0.15 and 1.0.