PURPOSE: To enable the addition and adsorption of ceramics to a natural fiber by immersing a natural fiber in a liquid composed of ceramics, an alkaline material, a nonionic surfactant and water and stored in a pressurizable and heatable tank.

CONSTITUTION: A natural fiber such as wool and cotton is put into a mesh bag and charged into an inner tank 35 formed of perforated plates. After closing the door of the tank, a driving motor 33 is actuated to move a cart 32 in the direction of the arrow A and the opening 31 of the 2nd tank 30 on the truck 32 is closed by closely contacting the 2nd tank with the opening part 11 of the 1st tank 10 consisting of a cylindrical vessel fixed to a base 1 with brackets 2. A pasty liquid composed of far-infrared radiation ceramic fine particles, an alkaline material, a nonionic surfactant and water is poured into the tank through a manhole 12 and the liquid level is controlled with a level controlling apparatus 20 while inspecting the level with a level gauge 19. Compressed air is supplied through a pressurizing port 14 and steam is supplied through a steam inlet port 21 to effect the pressurization and heating of the fiber. After adsorbing the ceramic fine particles to the swollen natural fiber, the supply of steam is stopped and the pressure is dropped to complete the treatment.