PURPOSE: A stable dispersion, obtained by using a copolymer prepared by copolymerizing a dicyclopentadiene derivative with an unsaturated monomer in the presence of a maleic acid-modified polybutadiene, and having improved stable drying property, water resistance, hardness, etc. without using an emulsifying agent.

CONSTITUTION: An aqueous resin dispersion containing a copolymer obtained by copolymerizing (B) 5W30pts.wt. dicyclopentadiene derivative expressed by the formula (R is H, lower alkyl or halogen; R' is bifunctional organic group; n is 0 or 1) and (C) 20W90pts.wt. another copolymerizable unsaturated monomer, e.g. an alkyl (meth)acrylate or styrene, in the presence of (A) 5W50pts.wt. polybutadiene, modified with maleic acid, and containing 50mol% or more butadiene component bonded with 1,2-vinyl bonds. The component (A) preferably has an acid value of 50W200 and a number-average molecular weight of aout 700W10,000.