A distillation apparatus for subjecting a crude readily polymerizable compound to distillation under vacuum conditions to purify the same includes a distillation column and a vacuum generator, an exhaust gas conduit of the vacuum generator being connected to a connecting conduit therebetween through a pressure control valve. The purification method includes using the disclosed distillation apparatus and controlling the action of the pressure control valve based on a pressure of the distillation column, thereby controlling the amount of the exhaust gas to be introduced to control the pressure of the distillation column.