The present invention provides a steel sheet surface treatment agent characterized in containing an aqueous urethane resin composition obtained by dispersing, in an aqueous medium (C), a urethane resin obtained by reacting a urethane prepolymer (A), which has an isocyanate group obtained by reacting a polyol (a1) and a polyisocyanate (a2) containing an aromatic polyisocyanate, with a chain terminator (B) containing an alcohol solvent (b1) with a boiling point in the range of 150-350 DEG C and a number average molecular weight of 500 or less. Said steel sheet surface treatment agent is able toform a coating film with excellent thermal yellowing resistance, chemical resistance and corrosion resistance and capable of preventing deterioration of various kinds of metal substrates.