A process for production of carboxylic acid polymers which comprises the first polymerization step (A) of introducing an aqueous monomer solution containing an α-unsaturated carboxylic acid or a salt thereof, an aqueous solution containing a hydrogensulfite salt, and an oxygen-containing gas into a flow mixer and polymerizing the monomer and the second polymerization step (B) of subjecting the reaction product of the first polymerization step containing unreacted monomer to additional polymerization. Even when the amount of an oxygen-containing gas used in initiating the polymerization is extremely reduced, high-quality carboxylic acid polymers which are little discolored and little contain impurities can be produced at high productivity.