PURPOSE: To reduce treatment unevenness by setting the temperature of the treating liquid to be brought into contact with a porous material at a uniform temperature always, while the amount of the treating liquid is remarkably reduced by a method in which the treating liquid is fed only to the inner tank filled with porous material, and the treating liquid is circulated in a state where the treating liquid in the inner tank is set at the prescribed temperature.

CONSTITUTION: The inner tank 22 filled with porous material 25 is changed in a treating tank 20. In this state, the treating tank 20 is sealed, and the pressure in the treating tank 20 is reduced by operating pumps 48, 51. When said tank becomes high vacuum-state, said state is kept for prescribed time, and then the treating liquid is fed into the inner tank 22 by operating the opening and closing valve of a feeding pipe arrangement 38. While the opening and closing valve of the feeding pipe arrangement 38 is closed at the stage where the porous material 25 has been dipped perfectly in the treating liquid, pressure-reduction is stopped, and by opening pressurizing valve 41 or 42, the pressure in the treating tank 20 is raised. Simultaneously with this operation, the treating liquid is circulated in the direction shown by an arrow A by operating a treatment liquid-circulating pump 35. The circulating treatment liquid permeates gradually through the porous material 25, whereby impregnation treatment is advanced. After the holding of said process for prescribed time, the pressure in the treating tank 20 is restored to normal pressure, and then when the treating liquid is removed, and the inner tank 22 is taken out, the impregnation-treated porous material 25 may be obtained.