A mixing device comprising: a mixer body comprising a cylindrical rotating member having a hollow cylindrical housing wherein the cylindrical housing is made of a metal sheet, the mixer body is rotatable around a center axis line of the cylindrical housing via a rotating drive shaft connected to a top plate that encloses an upper end of the cylindrical housing, and a suction port formed at a lower end of the cylindrical housing, wherein when the cylindrical rotating member rotates, the extruded plate portions cause an inner circulation current in which a liquid to be mixed circulates around the center axis line, and a portion of the liquid to be mixed, that forms the inner circulation current, is discharged outwardly as an outer discharge current (d1 - d4) by centrifugal force through the discharge ports at the same time when an outer portion of the liquid to be mixed that is outside of the cylindrical rotating member is sucked into the cylindrical rotating member as suction current via the suction port characterized in that portions of the metal sheet of the cylindrical housing are punched in the outer circumference thereof to form plate members and gap surrounding portions of the plate members, that the plate members are folded inward forming extruded plate portions (24A-24D) resulting in a plurality of discharge ports (22A - 22D) and that the plurality of extruded plate portions (24A - 24D) extend from the end edges of the discharge ports at the rotation direction side and protrude inwardly in a direction toward the central axis line. a mixer body comprising a cylindrical rotating member having a hollow cylindrical housing wherein the cylindrical housing is made of a metal sheet, the mixer body is rotatable around a center axis line of the cylindrical housing via a rotating drive shaft connected to a top plate that encloses an upper end of the cylindrical housing, and a suction port formed at a lower end of the cylindrical housing, wherein when the cylindrical rotating member rotates, the extruded plate portions cause an inner circulation current in which a liquid to be mixed circulates around the center axis line, and a portion of the liquid to be mixed, that forms the inner circulation current, is discharged outwardly as an outer discharge current (d1 - d4) by centrifugal force through the discharge ports at the same time when an outer portion of the liquid to be mixed that is outside of the cylindrical rotating member is sucked into the cylindrical rotating member as suction current via the suction port characterized in that portions of the metal sheet of the cylindrical housing are punched in the outer circumference thereof to form plate members and gap surrounding portions of the plate members, that the plate members are folded inward forming extruded plate portions (24A-24D) resulting in a plurality of discharge ports (22A - 22D) and that the plurality of extruded plate portions (24A - 24D) extend from the end edges of the discharge ports at the rotation direction side and protrude inwardly in a direction toward the central axis line. a cylindrical rotating member having a hollow cylindrical housing wherein the cylindrical housing is made of a metal sheet, the mixer body is rotatable around a center axis line of the cylindrical housing via a rotating drive shaft connected to a top plate that encloses an upper end of the cylindrical housing, and a suction port formed at a lower end of the cylindrical housing,