[Problem] To provide a novel antimicrobial agent that has excellent antimicrobial activity and which is prepared from an ester composed of propanediol and a β-hydroxycarboxylic acid.[Solution] An antimicrobial agent which is prepared from a propanediol mono-β-hydroxycarboxylic acid ester expressed by the following formula (1) or formula (2) (wherein R1 and R2 are both branched or unbranched, and saturated or unsaturated alkyl groups having 5 to 9 carbon atoms).