PURPOSE: To obtain an antimicrobial and mildew-proofing agent composition effectively acting on a wide range of cells, having high safety, by using an alkyldimethylaminoacetic acid betaine having excellent antimicrobial and mildew-proofing properties and a specific number of carbons and preferably a cationic surface active agent.
CONSTITUTION: An antimicrobial and mildew-proofing agent composition containing an alkyldimethylamino acid betaine shown by formula I (R is 14W16C alkyl or oleyl). Further the compound shown by formula I is blended with a cationic surface active agent, preferably a compound shown by formula II (R is 12W18C alkyl; R 1 WR 3 are methyl or benzyl; X is halogen) or a compound (e.g. laurylbenzyldimethylammonium chloride, etc.) shown by formula III to synergistically improve antimicrobial and mildew-proofing effects. The weight ratio of the cationic surface active agent/the alkyldimethylaminoacetic acid betaine is preferably 80/20W20/80.