The present invention provides a nanoporous fiber being substantially free from coarse pores and having homogeneously dispersed nanopores, unlike conventional porous fibers. A porous fiber has pores each having a diameter of 100 nm or less, in which the area ratio of pores each having a diameter of 200 nm or more to the total cross section of the fiber is 1.5% or less, and the pores are unconnected pores, or a porous fiber has pores each having a diameter of 100 nm or less, in which the area ratio of pores each having a diameter of 200 nm or more to the total cross section of the fiber is 1.5% or less, the pores are connected pores, and the fiber has a strength of 1.0 cN/dtex or more.