An electrically conductive composite fiber comprising an electrically conductive layer formed of a polyester-based polymer (A) having a melting point of 200° C. or higher and containing from 23 to 33% by weight of electrically conductive carbon black, and a protective layer formed of a polyester-based polymer (B) having a melting point of 210° C. or higher, wherein the difference between the SP value of the (A) and the SP value of the (B) is adjusted to not greater than a predetermined value and the fiber strength and the elongation at break are adjusted within certain ranges. This can make it possible to obtain an electrically conductive composite fiber that has a superior antistatic performance, which is not degraded very much over a practical wearing for a long term, though it contains only a relatively small amount of electrically conductive carbon black, and that is suitable for the field of clothing such as clean room wears and working wears.