To provide a liquid halogenated hydantoin microbicidal agent having an excellent growth inhibitory effect and microbicidal effect, especially on soil and slime disturbances occurring in process water of a paper, pulp manufacturer, industrial circulating cooling water or industrial waste water, and at places with a high humidity such as a bathroom, or kitchen, excellent in handleability, less decompositing of the halogenated hydantoin, excellent in product stability.

This liquid halogenated hydantoin microbicidal agent contains (A) ≥1 kind of halogenated hydantoin expressed by general formula (1) (wherein, at least one of X1, X2 is independently chlorine or bromine atom, and another one is any of chlorine, bromine or hydrogen atom; R1, R2 are each independently H or a 1-12C alkyl) and (B) ≥1 kind selected from pyrrolidones and γ-butylolactones as active ingredients.