To provide a sea-island type fiber enabling a waste fluid thereof to be easily treated, scarcely causing a load to the environment, and capable of forming a fine polyester having a uniform fiber diameter.

This sea-island type fiber is formed by using polybutylene succinate as a sea component and a polyester as an island component, wherein the polyester has a mass decreasing rate of ≤5 mass%, when measured under a condition of decomposing and removing the polybutylene succinate by a hydrolase. It is preferable that lipase is used as the hydrolase and a resin component selected from polyethylene terephthalate, polybutylene terephthalate, and polytrimethylene terephthalate is used as the island component.