1. An assertion description conversion device comprising: verification target identification unit parsing the syntax of a high-level assertion description described by a high-level assertion description language for verification of an inputted design description of a broader term to identify a verification target; design description searching unit searching a description on the verification target in a design description of a broader term; verification target description conversion unit converting the description of the verification target into a design description of a narrower term according to the design description of the narrower term corresponding to the description searched by the design description searching unit; verification content analysis unit for analyzing verification contents for the verification target described in the high-level assertion description; and verification content description conversion unit for converting the verification contents analyzed by the verification content analysis unit into the description format of a low-level assertion description language for verification of a design description of the narrower term.