PURPOSE: To suppress side reaction and enable obtaining of the subject compound useful as a raw material for flocculants, etc., in high quality and yield by reacting a vinyl monomer having tertiary amino group with a quaternizing agent in a mixed solvent of water and a specific alcohol.

CONSTITUTION: A vinyl monomer, expressed by the formula (R1 is H or methyl; R2 is 1-4C alkylene or 2-4C hydroxyalkylene; R3 and R4 are 1-4C alkyl, etc.; A is O or NH) and having tertiary amino group is reacted with a quaternizing agent (e.g. methyl chloride) in a mixed solvent of water and a secondary or tertiary alcohol to afford an unsaturated quaternary ammonium salt. Dimethylaminoethyl (meth)acrylate, etc., are cited as the compound expressed by the formula. Side reaction, such as hydrolysis or polymerization, can be effectively prevented by the aforementioned method.