Authors are developing a simulation program to evaluate the effect of preventive safety assistance systems. In the program, a driver model conducts brake operation to avoid a collision. In the process there are two factors to be determined; the braking timing and the brake pedal force after starting brake operation. In this paper, the algorithm for determining the brake operation start timing by use of the brake pedal force is examined. The time when the brake pedal force surpasses the threshold value is set as the start timing of brake operation. This algorithm was validated by the experimental data in the car following situation.