PURPOSE: To sufficintly clean the internal pores of a porous body so as to reach the interior of each fine pore, by such simple operation that a magnetic fluid is injected into the internal pores of the porous body and this injected magnetic fluid is moved by line of magnetic force to clean internal pores of the porous body.

CONSTITUTION: A magnetic fluid prepared by dispersing ferromagnetic particles throughout a liquid phase in a colloidal form is injected into the internal pores of a porous body and moved by applying a magnetic force thereto to perform the cleaning treatment of said pores. In order to move the magnetic fluid in the internal pores of the porous body, a rotary magnetic field may be applied or an AC magnetic field may be applied. As a result, the internal pores can be throughout cleaned at once. In addition, because impurities in the internal pores are also discharged along with the magnetic fluid, the magnetic fluid can be recovered and economical effieiency is also achieved because the magnetic fluid can be repeatedly used.