PURPOSE: To permit continuous and automatic operations from working to annealing treatment of a material to be annealed by placing the material to be annealed on a coolable slat conveyor consisting of a nonmagnetic material and running the conveyor in a cylindrical high-frequency heating coil.

CONSTITUTION: The material 3 to be annealed is placed on the slat conveyor and the conveyor is run in the high-frequency heating coil 2 of a high-frequency device provided with the heating coil 2 formed to a cylindrical shape. The slat conveyor is constituted of a chain 4 consisting of the nonmagnetic material and pipe-shaped chain rests 5 in which cooling water 6 is passed. The slat conveyor chain is thereby kept free from the influence of high frequencies and is cooled by the cooling water 6, by which the elongation by the heat is minimized. The working of the material 3 to be annealed, the automatic supply thereof continuously to the high-frequency heating furnace and the treatment thereof are thus permitted.