There is provided a superior decreasing method of an N-oxyl compound contained in an easily polymerizable material, according to which the N-oxyl compound contained in an easily polymerizable material can be sufficiently decreased and loss of the easily polymerizable material is little. The easily polymerizable material containing an N-oxyl compound and an acid are contacted with each other. In addition, not only the N-oxyl compound but also an organic titanium group compound contained in the easily polymerizable material can be sufficiently decreased.