Carbon membranes were continuously prepared by pyrolyzing an asymmetric polyimide hollow fiber membrane at temperatures ranging from 600 to 1000 degrees C under nitrogen for 3.6 min. The asymmetric polyimide hollow fiber membrane was treated under atmospheric air at 400 degrees C before pyrolysis. By scanning electron microscopy, the carbon membranes were observed to have an asymmetric structure of which a skin layer consisted of nodule aggregates. The asymmetric carbon membranes displayed high performance of gas permeability and selectivity. For example, the membranes prepared by pyrolyzing at over 700 degrees C had permeation rates of hydrogen ranging from 10(-4) to 10(-3) cm(3) (STP)/(cm(2) s cmHg) and ratios of the permeation rate of hydrogen to that of methane ranging from 100 to 630 at a feed gas composition of 50% hydrogen in methane and at 80 degrees C.