A gas/liquid mixing equipment of the present invention comprises a stirring vessel 1, a stirring shaft 10 inserted horizontally in the stirring vessel 1 and a helical ribbon impeller 20 attached to the stirring shaft 10, whereby a high gas absorption performance can be secured even with low shearing. Further, a polymer can be produced with high productivity. A gas/liquid mixing method of the present invention is a method which comprises employing the above-mentioned gas/liquid mixing equipment, and a method for producing a polymer of the present invention is a method which comprises polymerizing feed monomers containing gaseous monomers in aqueous solvents, wherein the gaseous monomers and the aqueous solvents are mixed by such a gas/liquid mixing method. A polymer of the present invention is produced by the above-mentioned method for producing a polymer.