Abstract: A cooling device for a false twisting machine having a fixed wall member 16 and movable wall members 18a and 18b which have faced side surfaces between which yarn treatment passageways 19a and 19b are formed. The facing side surfaces are formed with yarn contacting portions 16a1 and 16b1 and 18a-1 and 18b-1, which are spaced along the direction of the running of the yarn in the yarn treatment passageways. The arrangement of the yarn contacting portions are such that contact with the yarn occurs alternately between the fixed wall member 16 and movable wall members 18a and 18b. A sucking duct 14 is opened to the yarn treatment passageways 19a and 19b to generate flows of fluid transverse to the running direction of the yarn in the passageways 19a and 19b.