Islands-in-sea type composite fiber and process for producing the same

An islands-in-sea type composite fiber comprising a sea part comprising an easily-soluble polymer and a plurality of island parts comprising a hardly-soluble polymer, in a cross-sectional profile of which, each of the island parts has a thickness in the range of from 10 to 1000 nm, the number of the island parts is 100 or more per fiber and the intervals between the island parts adjacent to each other are 500 nm or less,

• wherein
• when four straight lines pass through the center of the cross-sectional profile of the composite fiber, at angular intervals of 45 degrees, the cross sectional thickness (r) of the island parts, and a smallest intervals (S min) between the island parts located on the four straight lines, and the cross sectional thickness (R) of the composite fiber and a largest intervals (S max) of the island parts located on the four straight lines satisfy the following requirements (I) and (II):
• 0.001≦S min/r≦1.0 (I)
• and
• S max/R≦0.15 (II).