PURPOSE: To prevent deterioration of a cationic vinyl monomer during production W during storage and stabilize the cationic vinyl monomer without impairing polymerization reactivity, by adding polycarboxylic acids to a cationic vinyl monomer at a definite amount and adjusting free amine concentration.

CONSTITUTION: At least one kind of compound (e.g. oxalic acid) selected from a group consisting of a polycarboxylic acid and salt thereof as a stabilizer is added to a cationic vinyl monomer expressed by the formula (R1 is H or methyl; R2 and R3 are 1W4C alkyl; X- is Cl- or 1/2 SO42-; n is 1W4). The addition amount is 1W5,000ppm based on the compound expressed by the formula. Further, free amine concentration and free acid concentration in the above- mentioned cationic vinyl monomer are each kept to ≤0.3wt.% and ≤1.0wt.% to stabilize the cationic vinyl monomer. The compound expressed by the formula is useful as a raw material monomer for cationic polymer useful as fiber modifier, water treating agent, paper chemical, medicine, cosmetic, etc.