There is provided a porous hollow fiber of vinylidene fluoride resin which has a water permeation rate that is large per fiber and little dependent on the length, has a large treatment capacity per volume of a filtering module, and is therefore suitable as a microfilter element. The computer receives an initial digital data set representing the patient’s teeth at their initial positions and a final digital data set representing the teeth at their final positions. 5 meqg in the molecule and having a number-average molecular weight of 15000 to 200000. The limiting disk has a plurality of positioning pivotal holes formed on the limiting disk near an edge of the limiting.