PURPOSE: To effectively prevent the deposition of polymer scales on the inside wall near the interface between the vapor phase and the liquid phase as well as the inside wall in the liquid phase.

CONSTITUTION: A polymer scale inhibitor to be used for polymerizing an ethylenically unsaturated monomer and comprising a condensation product obtained by adding a reaction stopper to a reaction system for obtaining a condensation product of an aliphatic diamine compound with a quinone compound and a process for producing a polymer in a polymerizer having a coating of the inhibitor on the inside wall.