This paper presents a headlamp model that can be used as part of a computer simulation of a wiring system in a vehicle. As more electrical equipment is installed in a vehicle, the wiring system connecting such equipment becomes more complicated, and thus it is essential to conduct a simulation of the wiring system to confirm its capacity. To perform this simulation requires exact models of the electrical equipment fitted to the vehicle. A headlamp model is especially important since currents flowing in the lamp vary dramatically. In this paper, a headlamp model is developed on the basis of the first law of thermodynamics, which is also applicable to a fog lamp. The headlamp is modelled as a variable resistor whose resistance is determined by the change in the operating temperature on account of the applied voltage. The resistance change was simulated using the model, and the resistance of an actual headlamp in a car was measured. Since the complete model is too complex, a simplified simulation model, suitable for use in a computer simulation, is also presented. Comparison of the two results showed good agreement, and through this simulation the reliability of the presented model was confirmed.