A carrier particle (A) is previously treated with a compound (B) such as a silane coupling agent, if necessary. To the carrier particle, a free functional group is introduced by allowing a reactive group of the carrier particle (A) to react with a compound (C) such as a bisoxazoline compound. As a result, a reactive particle represented by the following formula (I):[C--(B1).sub.m ].sub.n --A--[(B2).sub.p --D].sub.q (I)wherein A is a carrier particle, C is an organic group having at least one free functional group, D is a hydrophilic or hydrophobic group, B1 is an organic group connecting the carrier particle A to the organic group C and B2 is an organic group connecting the carrier particle A to the hydrophilic or hydrophobic group; n is an integer of not less than 1, m and p are 0 or 1, q is 0 or an integer of not less than 1, m and p may be different according to n and q; and having a number of active sites can be obtained. A hydrophilic or hydrophobic group may be introduced to the carrier (A).