Abstract: A cyclone is disclosed, which comprises a cylindrical shell, an inverted conical hopper depending from the cylindrical shell, a cylindrical dust exhaust duct leading from the lower end of the inverted conical hopper, a gas supply duct extending tangentially or circumferentially of the cylindrical shell for introducing dust-containing gas into the cyclone, and a gas exhaust duct penetrating the top wall of the cylindrical shell with the lower end of the gas exhaust duct positioned below the top wall, and in which the peripheral wall of the cylindrical shell has a projecting side wall portion inwardly projecting into the cyclone and terminating in the joint of the inner side wall of the gas exhaust supply duct and the peripheral wall of the cylindrical shell. The cylindrical gas exhaust duct has a protruding portion, which smoothly protrudes sidewise from its lower end and has a diameter equal to or greater than the diameter of the gas exhaust duct.