A simulation system for producing equivalent circuits reads data corresponding to a tabular W element format in a storage device, and adds data of the tabular W element format file using interpolation algorithm. A frequency-dependent transmission matrix is transformed into an N-port network matrix describing electrical properties of a multi-input and multi-output network. An N-port network matrix is transformed into a S-parameter matrix. A range of frequency of a s-parameter is determined and numbers of pole-residue, times for recursion and durable maximum system errors in the equivalent circuit is also determined. A vector fitting algorithm is performed and a rational function matrix composed with s-parameters is produced, to produce a general SPICE equivalent circuit based on the generated rational function matrix.