It is intended to provide a porous hollow fiber membrane for treating blood which suffers from little mechanical damage in the course of production, transfer or handling and shows excellent fractionation properties though it is a thinned polysulfone-based hollow fiber membrane. A porous hollow fiber membrane for treating blood which comprises a polysulfone-based polymer and polyvinylpyrrolidone and has a gradient porous structure with an increase in pore size from the inside to the outside along the membrane thickness direction, characterized in that the porous structure has a membrane thickness of 25 µm or more but less than 40 µm, and fibrils having an average diameter of from 100 to 200 µm are provided in such a manner that the ratio (To/Ti) of the average outside diameter (To) along the membrane thickness direction to the average inside diameter (Ti) is adjusted to not more than 2.
