At present the biodegradable polyesters such as polycaprolactone are expected to be utilizable for preventing the environmental pollution due to plastics waste. The aliphatic polyesters are truly biodegradable in soils, but they are also photodegradable. Molecular weight of polylactic acid decreases to the half value by 1h UV irradiation. The mainchain scission occurs via the Norrish II type photo cleavage. Polycaprolactone is also well photodegraded. From these results, the aliphatic polyesters are concluded to be utilizable for a photodegradable polymer.