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 file readout device which reads description contents of an inputted 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 according to an optimization rule; 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.