PURPOSE: To obtain surface-improved carbon fiber having specific elastic modulus, etc., by subjecting carbon fiber to electrolysis treatment in an aqueous solution of inorganic acidic electrolyte having specific pH, etc., making the carbon fiber as anode, then subjecting to electrolysis treatment in an aqueous solution of organic alkaline electrolyte containing organic amine and having specific pH, etc.

CONSTITUTION: Carbon fiber is subjected to electrolysis treatment in an aqueous solution of inorganic acidic electrolyte having pH<7 or neutral salt electrolyte making the carbon fiber as anode, then subjected to electrolysis treatment in an aqueous solution of inorganic alkaline electrolyte having pH>7, or neutral or alkaline ammonium salt, containing 0.5-10wt.% organic amine polymerizable by electrolysis to afford the surface-improved carbon fiber having excellent adhesion with matrix resin, ≤40t/mm2 elastic modulus, 0.6-1.4μA/cm2 ipa value by electrochemical measurement and 0.10-0.24 amount of oxygen-containing functional group (O1s/C1s) and 0.03-0.20 amount of nitrogen-containing functional group (N1s/C1s) on the surface of the carbon fiber by X-ray photoelectron spectroscopy.