With the recent, rapid developments of computers, adaptation between the human being and computer is being improved. On the other hand, mental fatigue is increased by continuous, long-time use. The problem of mental fatigue is now a new focus of research. This study proposes a method of evaluating mental fatigue. In this study, we used a method to extract signals related to fatigue using Independent Component Analysis (ICA). The subjects were 10 male students. After assuming a workload of simulated visual display terminal (VDT) operation for 30 min, the subjects sat in a relaxed position for 30 min. In this process, we measured work performance, subjective symptoms of feelings of fatigue and physiological measures. As a result, there was a high value of the correlation coefficient between these signals and the subjective symptoms of feeling of fatigue. We suggest that by using ICA we can determine the relationship between Independent Components and subjective feelings of fatigue for those under work stress, work strain and in a state of fatigue and stress. However, further experiments are needed to obtain conclusive findings of our research.