To provide a method for efficiently culturing the subject microorganisms after surely acquiring them.

This method for culturing mineral oil-degradative microorganisms comprises the following practice: from water and soil existing in the natural world, a plurality of materials, for example, sea bottom muddy sand bed and/or undersea drifting layer containing marine microorganism colonies, muddy effluent and/or river or marsh water containing terrestrial aquatic microorganism colonies, soil collected by digging the field or paddy and containing terrestrial soil microorganism colonies, are collected; each of these materials is spread on a medium made of a medium material containing a specified kind of mineral oil to conduct an initial culture of mineral oil- degradative microorganisms; thereafter a several kinds of the mineral oil- degradative microorganisms thus initially cultured are injected into and mixed with media each made of a medium material containing a specified mineral oil, respectively, to carry out an inertial culture, thus producing modified bacterial colonies.