To provide a cationic vinyl monomer which is stable with the passage of time and is free from the increase of its viscosity or the occurrence of popcorn polymerization due to exterior temperature, light, impact, a few amount of an ion of a metal such as iron in particular, during production, transportation or storage, and also exhibits satisfactory polymerizability.

The cationic vinyl monomer is prepared by adding 0.1 to 5,000 ppm of a thiol compound or a nitroxyl radical compound as a stabilizer, to a cationic vinyl monomer represented by general formula (1) (R1 is H or methyl group; R2 and R3 are each independently identically or differently a 1-3C alkyl group; R4 is a 1-3C alkyl group or a benzyl group; A is a 1-3C alkylene group; X is a halogen atom, a methylsulfonyl group, an ethylsulfonyl group or a p-toluene sulfonyl group).