A transistor model for a simulator simulates a resistance between a source region and a drain region with a model equation which has terms representing resistance values corresponding respectively to areas of mutually different impurity concentrations below a gate section in simulating characteristics of a transistor. At least two of the terms each having a threshold parameter indicating a voltage at which a semiconductor element composed of the associated region and regions adjacent to that region changes from an ON state to an OFF state. The threshold parameters of the terms being specified independently from each other. Thus, the characteristics of a transistor having a set of areas of mutually different impurity concentrations below a gate section, inclusive of subthreshold regions which are difficult to evaluate through actual measurement, can be simulated to high accuracy while preserving a good fit with a capacitance model.