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To quantify the effect of visually-induced motion sickness on the autonomic nervous system, the authors previously proposed a new physiological index rhomax representing the maximum cross-correlation coefficient between blood pressure variability and heart rate variability whose frequency components are limited to the Mayer wave band. However, rhomax requires measurement of continuous blood pressure with an expensive and bulky measuring device. In the present study, an easier method for obtaining rhomax with measurement of neither continuous blood pressure nor ECG but using finger photoplethysmography (PPG) only has been developed. A blood pressure-related parameter was extracted from the PPG by using the independent component analysis. Two experiments in which subjects performed the Valsalva maneuver and then they watched a swaying video image were carried out to evaluate the adequacy of the proposed method. The experimental results have shown that the proposed method worked successfully as good as the conventional method. This means that the proposed method can contribute to increase in the number of subjects because multiple subjects can be used even in a single experiment.