One of the solutions to the problem of marine plastic pollution is utilization of marine biodegradable plastics. Evaluation of biodegradability of a plastic material in marine is essential for developing marine biodegradable plastics. Most of plastic waste tend to sink and lay on the sea floor due to degradation and adhesion of micro-organisms after floating in seawater. In this study, the effect of three exposure conditions, i.e. in seawater, in seawater/sediment interface, in sediment, on the biodegradability of the plastic material was evaluated. As a result of a disintegration test, it was shown that the exposure conditions were one of the factors affecting the biodegradation process and biodegradation rate of the plastic material. Furthermore, the biodegradability tests using the same inoculum revealed that the biodegradation rate of the plastic material in the seawater/sediment surface was the fastest among the three conditions. Even if the test is performed in the same exposure condition, the biodegradation rate varied depending on the season of the exposed sea area. Therefore, it is necessary to simulate various marine environmental conditions for the evaluation of biodegradability of plastic materials in marine.