The object of the present invention is to restore soil and prevent contamination of ground water by decomposing an organic compound with decomposing microorganisms in soil contaminated with an agricultural chemical or other organic compound. The present invention discloses an accumulation method for isolated decomposing microorganisms comprising: accumulating decomposing microorganisms of a species able to decompose a specific organic compound in a porous material that is able to adsorb said organic compound while also having a countless number of pores that facilitate habitation by said decomposing microorganisms; wherein, by inoculating only said decomposing microorganisms into a porous material, forming an accumulation layer composed of that inoculated porous material, and perfusing that accumulation layer with an inorganic salt medium having only said organic compound for its carbon source and nitrogen source, said decomposing microorganisms are accumulated in the porous material in a state of high density, and a decomposing microorganism retentive carrier obtained in this manner is buried directly in contaminated soil.