The islands-in-sea type composite fiber of the present invention comprises a sea part containing an easily soluble polymer and 100 or more island parts containing a hardly soluble polymer, per fiber. In a cross-sectional profile of the composite fiber, each of the island parts has a thickness in the range of from 10 to 1,000 nm and the intervals between the island parts adjacent to each other are 500 nm or less. The islands-in-sea type composite fiber is produced by melt spinning the sea part polymer and the island part polymer mentioned above through a spinneret for an islands-in-sea type composite fiber and taking up the spun fiber at a speed of 400 to 6,000 m/min. Dissolution and removal of the sea part polymer from the composite fiber gives a group of fine fibers having a thickness of 10 to 1,000 nm and useful for clothing, industrial materials and other applications.