NOVELTY - A microorganisms decomposing-treating method of dioxins contaminated solid substance involves pretreating solid substance, mixing contaminant sludge, microbial cells of decomposing microorganisms and culture medium in reactor (2) and decomposing dioxins, separating sludge into culture solution and solid substance containing microbial cells and culture medium, and recycling of microbial cells and culture medium.    USE - For microorganisms decomposition of dioxins contaminated solid substance such as soil, reclamation material, incineration ashes and fly-ash.    ADVANTAGE - The method enables to safely and reliably decompose dioxins in a reactor. By returning the culture solution to the reactor and recycling microbial cell and culture medium, the nutrient of culture medium is perfectly utilized and the amount of culture medium supplied to the reactor is reduced. By maintaining the high concentration of microorganisms in the reactor, the decomposition efficiency of dioxins is increased. By returning the waste-water after washing solid substance, to the reactor, the residual nutrient of the culture medium is effectively utilized.    DETAILED DESCRIPTION - A solid substance contaminated by dioxins, is pretreated and the inhibitor which inhibits decomposition of dioxins by microorganisms, is removed. The contaminant sludge which removed the inhibitor, microbial cells of decomposing microorganisms and a culture medium are mixed, stirred in a decomposition reactor (2) and dioxins are decomposed by microorganisms. The sludge ejected from the reactor is separated into culture solution and solid substance containing microbial cells and culture medium, with a solid-liquid separator (4). The culture solution is returned to the reactor and circulation recycling of microbial cells and culture medium is performed.    DESCRIPTION OF DRAWING(S) - The figure shows the flow sheet for microorganisms decompose-treating method of dioxins contaminated solid substance. (Drawing includes non-English language text).    decomposition reactor (2)    solid-liquid separator and washing apparatus (4)