PURPOSE: A specific resin for sheathing and another resin containing high-density substance for core are extruded through a double dice into fiber and drawn to produce cored, high-density fiber of high strength.

CONSTITUTION: (A) A resin to form the sheath part of conjugate fiber by drawing at a prescribed temperature such as polyethylene and (B) another resin of high-density that is prepared by mixing a high-density powder 3 such as barium oxide with a resin that softens or keeps melting state at the drawing temperature of component A such as modified polyethylene are extruded into conjugate lines having the core of high-density resin 2 covered with the sheath resin 1. Then, the resultant lines are drawn at a prescribed temperature to produce the objective fiber with high-density core covered with high-strength sheath.