To obtain biodegradable fiber which has high biodegradability, practi cal levels of mechanical strength and heat resistance and is useful as a fiber material for industrial purpose, for example, in fishery, agriculture and civil engineering.

A resin composition comprising (A) 100 pts.wt. of an aliphatic polyester, preferably a polyester from succinic acid and 1,4-butanediol and (B) 1-200 pts.wt. of polycaprolactone is melt-extruded into a biodegradable fiber with a tensile strength of ≥2.0g/d. The component B induces the biodegradation of the component A, and a biodegradable fiber which can readily replace the conventional polyesters with itself in the application without any trouble is obtained at a relatively inexpensive cost.