Development of simulation-based evaluation and analysis support system for man-machine interface design (SEAMAID) has been conducted in the Nuclear Power Engineering Corporation to simulate the behavior of a few operators and the human-machine interface (HMI) in a commercialized pressurized water reactor plant. The workload is one of the key factors with respect to reducing the human error in the operation of nuclear power plants. In order to produce a high-quality design of HMI, the evaluation method was developed to simulate and analyze the operator's workload. Our method was adopted from the cognition model proposed by Reason. The workload such as the length of the visual point movement and the moving length of the operators was visualized in a monitor image during the simulation, and then recorded as a movie-file. As a consequence, the validation of SEAMAID was clarified.