1. A file conversion device which converts a file form written in a given language to a file form that can be interpreted by another language, the device comprising: a memory; and a processor configured to execute: a file input section which inputs a first file written in the given language; a job network creating section; and an actual processing line conversion section, wherein the job network creating section comprises: a file readout device which reads description contents of the inputted first file by each line, and manages the description contents by adding an index to each line of the read description contents; a syntax tree creating device which converts the description contents into a syntax tree of a tree structure based on the indexes through linearly scanning the indexes of the file managed by the file readout device; a syntax tree optimizing device which optimizes the tree structure of the syntax tree as a flowchart by searching divergence points according to a parallel execution instruction or an order control instruction of the syntax tree from an end of the converted syntax tree, judging whether or not nodes diverged at the branching points are connectable, and connecting the nodes when the nodes are connectable; and a file dividing device which divides the inputted file by having controllers in the flowchart optimized by the syntax tree optimizing device as identifiers, wherein the actual processing line conversion section converts a language of an actual processing description written in the files divided by the file dividing device into a command interpreter language.