To shorten the converting process time by successively converting a graphic data file which is divided into many parts.
A process part which converts graphic data into numeric control data and a storage part having recorded a converting program, the graphic data file which is divided into many files, and a file name list file wherein a list of the names of the graphic data files is recorded are provided, and a retrieving process for retrieving a graphic data file matching a file name recorded in the file name list file among the graphic data files and a converting process for generating a numeric control data file by converting the graphic data of the graphic data file matching the file name into numeric control data are repeated successively until there is no unconverted graphic data file recorded in the file name list.