1. A design description rewriting device, comprising: a first inputter configured to input design description of software or hardware wherein the design description includes a plurality of functions; a second inputter configured to input a set of type specifying description corresponding to the functions wherein each type specifying description specifies a type for a return value of each corresponding function, and the type specifying description of at least one function of the functions additionally specifies a type for a return value of a first function which is an other function of the functions; a determiner configured to determine a type of the return value of the first function to a comprehensive type to comprehend the types specified by the type specifying description corresponding to the first function and the at least one function and a type of the return value of a second function which is a function than the first function out of the functions to the type specified by the type specifying description corresponding to the second function; and a rewriter configured to rewrite the design description so that types determined by the determiner are set for the return values of the first function and the second function.