To treat organic sludge while keeping facility costs, cement production efficiency, and a reduction in clinker production amount to a minimum. An organic sludge treatment device includes: a fractionation device 7 that fractionates a preheated raw material R2 from a preheater cyclone 4C excluding a bottommost cyclone of a cement burning device 1; a mixing device 8 that mixes an organic sludge S with the fractionated preheated raw material, and that dries the organic sludge using sensible heat of the preheated raw material; and a supply device (mixture chute 12, double-flap damper 13, shut damper 14) that supplies a mixture M from the mixing device to a calciner furnace 5 of the cement burning device or to a duct disposed between a kiln inlet portion of a cement kiln 2 and the calciner furnace. The treatment device may be provided with an introduction device for introducing an exhaust gas G2 including dust, odor and water vapor from the mixing device to a gas outlet of a bottommost cyclone 4A of the cement burning device.