A circuit simulation apparatus and a modeling method are provided which are useful to design an integrated circuit in a very fine manner by forming a model of such a transistor that widths of element isolating-purpose insulating films are different from each other. In an isolation width depending parameter correcting means 4 of the present invention, an approximate expression of a parameter having an element isolating-purpose insulating film width depending characteristic is formed, and a value of a corrected parameter obtained by employing the formed approximate expression is replaced by a value of an original parameter, so that a transistor model of such a transistor is formed in which element isolating-purpose insulating film widths are different from each other. As a consequence, circuit simulation can be carried out in high precision by considering a change in transistor characteristics caused by a stress, which are approximated to actually measured data.