To obtain an extremely stable composition in which microbicidal activity of 3-isothiazolone is retained by formulating an aqueous solution of 3-isothiazolone compound with a specific unsaturated cyclic iocicsyl or iodyl compound.

This composition comprises (A) water, (B) a microbicidally active 3-isothiazolone compound (preferably 5-chloro-2-methyl-3-isothiazolone) and (C) an effective stabilizing amount of unsaturated, cyclic iodosyl or iodyl compound (preferably 2-iodosyl-1,4-benzenedicarboxylic acid) of formula I or II {X is C(O), SO2 or the like; Y is O or NR3 [R3 is H, a 1-18C alkyl or the like); R1 is H, Cl or the like; R2 is H, SO3H or the like; R8 to R10 are each CN, OH or the like; (n) is or 1]. It is preferable that the components B, C are each contained in amounts of 0.5-14 wt.% and 0.1-10 wt.%. Furthermore, when the concentration of the component B is 5.1-25 wt.%, the component C is preferably used in an amount of 2-10 wt.% and the concentration of the component C is 0.5-5 wt.%, the component C is preferably used in an amount of 0.1-7 wt.%.