PROBLEM TO BE SOLVED: To provide a method for purifying a readily polymerizable compound containing a polymerizable double bond in the molecule such as acrylic acid, methacrylic acid, an acrylic acid ester, a methacrylic acid ester, styrene, etc., efficiently, safely and inexpensively by distillation under reduced pressure. SOLUTION: In distilling a liquid containing a readily polymerizable compound such as acrylic acid, methacrylic acid, their esters, styrene, etc., a cooling/ condensing apparatus for vapor at the top of a distillation column is installed so as to make its condensation face vertical and a polymerization inhibitor is made to exist on the condensation face. Effectively a solution containing the polymerization inhibitor is sprayed on the top of the condensation face of the cooling/condensing apparatus.