PURPOSE: To efficiently isolate the subject bacteria for biodegradable plastics compared to such conventional methods as to take samples randomly.

CONSTITUTION: The objective isolation is conducted using samples taken from the soil or water having been brought into contact with plastics. Specifically, (1) a sample taken from plastic-embedded soil or plastic-set water is put to enrichment culture in a medium containing the plastic alone as carbon source; (2) as necessary, the above enrichment culture is repeated to thicken the aimed plastic-degradative bacteria; (3) the resultant culture fluid is spread on a solid medium to conduct an isolation culture; and (4) for the colonies obtained by the isolation culture, the degradation of the plastics is confirmed and the objective plastic-degradative bacteria is obtained.