To provide a copolymer for aqueous primer and an aqueous primer composition having water-resistant property, water-preventive property and adhesive property (substrate reinforcing property, durability).

The copolymer for aqueous primer includes at least the following monomer units (1) to (3), wherein the ratio of mass of (1) styrenes to (2) (meth)acrylates is 5/95 to 60/40. Therein, (1) is styrenes, (2) is (meth)acrylates and (3) is a cationic monomer having a specified structure.