PURPOSE: To obtain the subject modifying agent effective in improving the adhesivity of carbon fiber to a matrix resin by forming a polyimide resin composed of a mixed diamine consisting of two specific kinds of primary diamines and an aromatic tetracarboxylic acid on the surface of a reinforcing carbon fiber.

CONSTITUTION: A mixed diamine obtained by mixing a symmetric meta- substituted primary diamine of formula [X is O, SO2, S, C=O, CH2, C(CH3)2, C(CF3)2 or direct bond] with a symmetric aromatic para-substituted primary diamine at an equivalent ratio of (10-90):(90-10) is made to react with an aromatic tetracarboxylic acid dianhydride in nitrogen atmosphere to obtain a polyamic acid solution. A carbon fiber bundle is immersed in a liquid produced by diluting the above solution, dried with heat and heated in nitrogen atmosphere to convert the polyamic acid into polyimide and obtain a reinforcing carbon fiber having polyimide resin formed on the surface of carbon fiber. The obtained fiber has improved adhesivity to a matrix resin, especially thermoplastic resin and gives a molded article having remarkably improved mechanical strength by mixing with the resin and molding the mixture.