A porous multilayered hollow fiber for outer side pressed filtration is
made of a multilayer comprising a supporting layer constituted of the
thickness of an expanded porous polytetrafluoroethylene tube, and a
filtration layer provided on the outer surface of the supporting layer,
wherein pores surrounded by fibrous frameworks in the filtration layer
are smaller than those of the supporting layer, and the mean of the
maximum fibril length (L) of the fibrous frameworks surrounding each pore
in the outer surface of the filtration layer is designed to be small such
that (X) and (Y) fall within the range on the XY plane, where (X) is a
particle diameter of particles captured when the particle trapping ratio
is equal to or more than 90% in the case where the filtration is done
under an elevated pressure of 0.1 MPa, and (Y) is a value designated as
RFL obtained by dividing (L) by (X). Thus, the degradation of flow rate
due to the elapse of time can be prevented, since solid particles to be
separated do not enter into pores, and clogging and blockade of the pores
do not occur even in the case of filtering a solution of high turbidity.
As a result, it is possible to recover filtration property easily by
performing back-wash reverse filtration and to maintain excellent
filtration property for a long time.