An object of the present invention is to provide a sizing agent composition that gives a carbon fiber from which a carbon fiber-reinforced composite material having excellent adhesion between a resin and the carbon fiber and having excellent mechanical properties can be formed. The sizing agent composition of the invention is a sizing agent composition comprising (A) a blocked isocyanate, and (B) a compound containing at least one polar group and at least one unsaturated group per molecule. In the invention, the mixing ratio (mass ratio) of the blocked isocyanate (A) and the compound (B) containing at least one polar group and at least one unsaturated group per molecule (A/B) is preferably 95/5 to 5/95. In the invention, the blocked isocyanate (A) is preferably a compound having an aliphatic skeleton.