Provided is a novel layer migration control agent that is added to a paint composition when forming a multilayer coating film on a coating object by curing a prime coating film and a top coating film at the same time, the agent making it possible to control layer migration of the two coating films and obtain the multilayer coating film with satisfactory finish appearance. The layer migration control agent is a copolymer obtained by copolymerization of a monomer mixture including 5% by weight to 90% by weight of a polymerizable unsaturated monomer including an ether group and 10% by weight to 95% by weight of a polymerizable unsaturated monomer having a hydrophobic group having 1 to 22 carbon atoms, the copolymer having a weight average molecular weight of 1000 to 100,000.