Mental stress refers to the feeling of strain and anxiety caused by internal and/or external factors. Stress may have both positive and negative influences on cognitive performance. For example, small amounts of stress may improve concentration, athletic performance, and reaction speed. However, excessive stress may harm health, and disturb concentration and motor control. Furthermore, increase in short-term stress has a considerable effect on physiological processes such as heart rate and sweating. However, to our knowledge, so far no study has estimated mental stress through continuous monitoring of physiological signals under a situation with various stressors. Therefore, it remains unclear how different physiological signals correlate with each other and how they change continuously in response to the changes in stressors. Here, we measured heart rate variability, galvanic skin response, and activity of the masseter muscle in a professional racer during a real car race. Car racing is one of the most dangerous sports, and the competition with other racers and physical discomfort during the race induce great short-term stresses. We used factor analysis to examine the relation between the three types of physiological signals, and clarified the events associated with stress (e.g., acceleration, competing car in vision, overtaking). The results showed that the heart rate variability and galvanic skin response correlated with each other, and were associated with the event of competition, which brought great mental stress. In contrast, activity of the masseter muscle did not correlate with the other two physiological signals, but was associated with the events of acceleration or deceleration, which brought great physical discomfort. We concluded that heart rate variability and galvanic skin response reflect mental stress, which is associated with an internal desire to win and is triggered by changes in the external world (e.g., the appearance of competing cars). In contrast, masseter muscle activity reflects the endurance of body discomfort, which is not linked to any internal state. Our results indicated that internal and external stressors may be dissociated from each other, and may affect different organs and physiological receptors.