A nanoporous fiber wherein nanopores substantially not containing any bulky pores are uniformly dispersed unlike conventional porous fibers. In particular, a porous fiber having pores of 100 nm or less diameter characterized in that the area ratio of pores of 200 nm or greater diameter based on the entirety of fiber cross section is 1.5% or less and that the pores are present as closed cells; or a porous fiber having pores of 100 nm or less diameter wherein the area ratio of pores of 200 nm or greater diameter based on the entirety of fiber cross section is 1.5% or less, the pores being present as interconnected cells, which porous fiber has a strength of 1.0 cN/dtex or greater.
