Biodegradation behavior of test pieces of some biodegradable polymers formed by injection molding was investigated in soil, aqueous environments, and an activated sludge. The manner of the biodegradation depended on the composition of materials. With regard to the tests in soil and aqueous environments in the field, the degradation progressed during spring and fall. In an activated sludge of a sewage treatment plant, Biopol and Mater-Bi were degraded. The extent of the degradation of Biopol was influenced by the density of microbes. The degradation of Biopol was facilitated in an aeration tank, in which the microbe density was high. It was estimated that about 50 wt% of the biodegradable polymers was degraded in one year in the activated sludge. Waste treatment of polymers in a sewage treatment plant would be one of the effective methods, provided the safety of the treatment is confirmed.