To introduce carboxyl groups into an object body in high levels by applying an energy to a system in such a state as to be in mutual contact between the object body and a solution containing compound(s) such as a halogenoacetic acid.

Carboxyl groups are introduced into an object body by applying an energy to a system in such a state as to be in mutual contact between the object body and a solution containing at least one compound selected from halogenoacetic acids, salts thereof, halogenopropionic acids and salts thereof; wherein when the object body is a porous body, the above solution 2 is put into a reaction vessel 3 and a material 5 (feeding material 5) containing the solution 2 and capable of feeding the porous body 1 with the solution 2 is placed at the center of the vessel 3; by placing the porous body 1 to be modified on the feeding material 5, a contact between the solution 2 and the porous body 1 can be attained; the concentration of the compound(s) in the solution is pref. 0.5-10 wt.%.