For efficient exercise, motor functions, heart rate, oxygen uptake (which are controlled by autonomic nervous system activity), heat acclimation-related functions, such as sweating, and thermoregulation must work properly during exercise. In this research, a noncontact method of measuring capillary contraction and dilation, one of the autonomic nervous system activities, using only a commercial web camera was developed. The absorption rate by haemoglobin in blood differs for each wavelength of light. When the capillaries in the face contract or dilate, the colour component of the light reflected from the face changes. The focus of this study was on the changes in the green and blue values of the face image. Green light reaches the dermis, where capillaries are located, while blue light reaches only the epidermis. The G/B ratio, the green value divided by the blue value, shows the changes in capillary contraction and dilation. An experiment was conducted to validate the G/B ratio method. Ten subjects (23 ± 1.6 years of age) participated in the experiment, and face movement and heart rate were measured during an aerobic bike exercise test. The results showed that, when the heart rate increased, the G/B ratio decreased immediately after the start of exercise. After the exercise stopped, the heart rate decreased immediately, and the G/B ratio increased. The G/B ratio revealed that the sympathetic nervous system became dominant during exercise, causing facial capillaries to dilate, and that the parasympathetic nervous system became dominant after exercise, causing facial capillaries to constrict.