To provide a processing method for hollow fibers forming enough size and numbers of continuous holes to introduce liquids including a function affording substance, especially introducing liquids having viscosity ≥30 centipoise, into the cavities of the hollow fibers and greatly suppressing the decrease of the physical properties such as strength or the like, without requiring a great deal of expense.

The continuous holes along the longitudinal direction of the fibers from the surface of the fibers to the voids are formed by repeatedly adding stress within the elastic limit of the fibers, in the direction crossing with the longitudinal direction of the fibers at the lower temperature of both of the kick-off temperature or thermosoftening temperature over the glass transition temperature of the hollow fibers having at least 20% of percentage of hollowness. The processing time is shortened by carrying out the application of the stress to the hollow fibers in a buffer solution of pH 9.5 to 11.5.