A verilog-HDL source at the register-transfer level (RTL) is converted into a programming language executable on computer. Constructed in an analyzing of elements is a data structure corresponding to the elements of the verilog-HDL source. Created in an analyzing of a data-flow are a first data flow from a state register and a second flow from data-path register. Reconstructed in a reconstructing of a control-structure is the first data flow. Reconstructed in a reconstructing of a data-path is the second data flow so that the reconstructed second data is constituted only by circuitry operating in each state of the control structure. Each reconstructed data flow is mapped in each state of the control structure in a combining of the control-structure/data-flow, to output an behavior-level intermediate language. The intermediate language is converted into a programming language in a generating of an object-code.