To safeguard the environment without generating contaminated material by decomposing animal body and plant body containing metals with microorganisms, vaporizing the metals from the decomposed residue and recovering.

In a decomposing means 11, the animal body and the plant body containing the metals are decomposed with microorganisms. A large part of treated object is decomposed into water and carbon dioxide with the microorganisms. The carbon dioxide is gasified and diffused outside the system. The vol. of the treated object is remarkably reduced by vaporizing the water. The metals in the animal body and the plant body exist in the concentrated state in the decomposed residue. The decomposed residue is introduced into an airtight vessel of a recovering device 12. Scallop body contains high concns. of cadmium and arsenic but these metal compounds are decomposed with the microorganisms to recover the metals and therefore, the environment is not contaminated.