To provide a method for purifying an N-alkyl-substituted imide group-containing alicyclic olefin monomer which is a raw material monomer for a polar group-containing cyclic olefinic polymer useful as a sealing material and an insulating material for various optical parts and electronic parts.

There is provided the method for purification by which the N-alkyl-substituted imide group-containing alicyclic olefin monomer which is N-(2-ethylhexyl)-bicyclo[2.2.1]hept-5-ene-2,3-dicarboximide (NEHI) as a representative example is brought into contact with a mixed ion exchange resin comprising a cation exchange resin and an anion exchange resin. The ratio of the cation exchange resin and anion exchange resin in the mixed ion exchange resin is preferably (1:1) to (10:1) in terms of weight ratio of the cation exchange resin:anion exchange resin. The monomer is preferably brought into contact with the mixed ion exchange resin under conditions of 10-100°C.