To provide the method for decomposing various organic materials, oils or fats, hydrocarbons or the like by microorganisms under low temperature environment.

In a culturing decomposition tank 1, microorganisms having active growing power and decomposing power even in a state of 10°C or below, or particularly, in an extreme state of 0°C or below is mixed with a carrier 2, and the mixture is mixed with materials to be decomposed, such as organic materials or hydrocarbons to subject the materials to be decomposed to decomposing treatment under room temperature or low temperature environment. The carrier 2 is obtained by chaff, sawdust, pulp refuse, other organic materials, sand, pebbles, other inorganic materials, and combination thereof.