PURPOSE: A high-melting polymer in an oriented state and another low-melting polyester are subjected to conjugate spinning so that its cross section circumference ratio reaches a specific value, then drawn and heat treated at a specific temperature between melting points of both polymers to produce the titled fiber of good dyeability, high pilling resistance, heat resistance and tenacity.

CONSTITUTION: (A) A polymer that melts at 200W350°C and is in an oriented crystalline state such as polyethylene terephthalate and (B) another polyester polymer that melts at 100W230°C, more than 20°C lower than polymer A such as polybutylene terephthalate copolymer containing isophthalic acid are subjected to conjugate spinning so that the cross section circumference ratio of component B becomes 40%, and drawn to prepare oriented mixed spun yarn. Then, the resultant yarn is heat treated at a temperature more than 20°C lower than the melting point of component A and higher than that of component B to produce the objective conjugate fiber with component B unorieinted.