An antimicrobial composition which comprises an isothiazolone compound, suppresses decomposition of the isothiazolone compound and reduces irritation of skin caused by the isothiazolone compound is provided. The antimicrobial composition comprises an isothiazolone compound of the following formula [1] or formula [2] and an aminocarboxylic acid of the following formula [3] or a derivative thereof: ##STR1## wherein R.sup.1 is hydrogen, alkyl, alkenyl, alkynyl or aralkyl; X and Y are each hydrogen, halogen or form a benzene ring with the carbon atoms at the 4- and 5-positions of the isothiazolone compound; M is a cation of an alkali metal, alkaline earth metal, heavy metal or amine; Z is an anion; a is 1 or 2; n is an integer; R.sup.2 is hydrogen or alkyl; R.sup.3 is C.sub.1 -C.sub.5 alkylene; R.sup.4 is hydrogen or C.sub.1 -C.sub.5 alkyl; R.sup.2 and R.sup.4 form a heterocyclic group; and b is 0 or 1.