To provide ultrafine fiber good in feeling of hand touch when used in artificial leather or the like and good in color developability as well.

This joining-type conjugated staple fiber is such that a polymer A and a polymer B are alternately arranged on the fiber cross section and the mass ratio A/B is (90:10) to (10:90). After splitting this fiber, flat fibers consisting of the polymer A and the polymer B respectively are formed, having the following characteristics: each of the cross section thicknesses is ≤3 μm, the flatness ratio L/D is ≥2 (L and D are the length and the thickness of each of the cross sections respectively), and the fineness of each of the flat fibers is ≥0.02 dtex.