WE CLAIM:
1. A porous fiber comprising a modified solid cross-section, wherein the porous
fiber satisfies the following (a) to (b):
(a) a modification degree Do/Di, in a cross section of the solid fiber, is 1.20 to 8.50 when the inscribed circle diameter is denoted by Di and the circumscribed circle diameter is denoted by Do; and
(b) a porous specific surface area of the fiber is not less than 3 m2/g.

2. The porous fiber according to claim 1, wherein an average radius of pore is not less than 0.8 nm and not more than 90 nm.
3. The porous fiber according to claim 1 or 2, wherein the porous specific surface area is not less than 30 m2/g.
4. The porous fiber according to any one of claims 1 to 3, wherein an inscribed circle occupancy of the following equation is not less than 0.10:
Inscribed circle occupancy = Area of the inscribed circle of the cross section of the fiber / Area of the cross section of the fiber.
5. The porous fiber according to any one of claims 1 to 4, wherein a diameter of pore in the porous fiber is not more than 25 urn and a ratio of an average diameter of pore in the area adjacent to the outer surface of the fiber vs an average diameter of pore in the central portion area of the fiber is not less than 0.50 and not more than 3.00.
6. The porous fiber according to any one of claims 1 to 5, wherein the porous fiber has a structure of the fiber cross-section comprising a porous portion which has a network structure composed of a communication hole and a dense layer which has a more dense structure in comparison to the porous portion, and wherein the porous fiber satisfies the following (d) to (e):

(d) the porous portion and the dense layer are continued with each other; and
(e) the dense layer is located closer to the area adjacent to the outer surface of

the fiber than the porous portion and a distance Tl from the outermost surface of the fiber to the porous portion is not less than 0.001 urn and not more than 30 urn.
7. The porous fibers according to any one of claims 1 to 6, when a circle
equivalent diameter (urn) is denoted by T2, Tl and T2 satisfy the following equation:
Tl/T2< 0.030.
8. The porous fibers according to any one of claims 1 to 7, wherein the solid fiber is in a straight form.
9. The porous fibers according to any one of claims 1 to 8, wherein the circle equivalent diameter T2 is not less than 10 urn and not more than 1,000 urn.
10. The porous fibers according to any one of claims 1 to 9, wherein an open pore ratio at the surface of the porous fiber is not less than 0.5% and not more than 30%.
11. The porous fibers according to any one of claims 1 to 10, wherein a pore radius distribution index is not less than 1.0 and not more than 2.8.
12. The porous fibers according to any one of claims 6 to 11, wherein both the porous portion and the dense layer contain not less than 45 vol% of a material common to both as a component material.
13. The porous fibers according to any one of claims 1 to 12, wherein the porous fiber has a negative charge.
14. The porous fibers according to any one of claims 1 to 13, wherein the porous fiber comprises an amorphous polymer material.
15. The porous fibers according to any one of claims 1 to 14, wherein the porous fiber comprises an amorphous polymer material and the amorphous polymer material comprises a polymer with an ester group.
16. The porous fibers according to any one of claims 1 to 15, wherein the number of adhered platelets which are brought into contact with the surface of the porous fiber is not more than 30/(4.3 x 103um2).
17. An adsorbent material, comprising not less than 28 vol% of the porous fiber

according to any one of claims 1 to 16 as a fiber bundle.
18. The adsorbent material according to claim 17, wherein the adsorbent material is for use in a medical application.
19. The adsorbent material according to claim 18, wherein the adsorption amount of (32-microglobulin per fiber volume is not less than 0.005 mg/cm3.
20. A purification column which is formed by arranging the adsorbent material according to any of claims 16 to 19 in a straight form in an axis direction of a plastic casing and by attaching an inlet port and an outlet port of a fluid that is to be treated to both ends of the plastic casing.