The understanding and treatment of pain is one of the oldest challenges in the field of clinical medicine. In this study, as a first step toward adequate pain assessment, we propose a method to evaluate the reactions of the automatic nervous system in response to painful stimuli by observing arterial wall impedance. Under the proposed method, the mechanical impedance (stiffness) of the arterial wall is calculated from blood pressure and photoplethysmogram measurements on a beat-to-beat basis. In the experiments, we tested eight male subjects (aged 22-23) by applying external forces (1-3 [N]) to the central parts of their palms as painful stimuli, and evaluated changes in levels of arterial wall stiffness during stimulation. The results indicated that stiffness during stimulation showed a significant increase (p = 0.007, p = 0.014 and p = 0.018 for the stimulus changes from 0 to 1 [N], 1 to 2 [N] and 2 to 3 [N] for all subjects). We also compared the coefficients of variation in the measured stiffness and visual analog scale (VAS) values during stimulation, and found that the mean coefficients of variation for stiffness (0.37, 0.27 and 0.26 for the stimuli of 1, 2 and 3 [N] for all subjects, respectively) were smaller than the ones of the VAS values (0. 67, 0.51 and 0.50, respectively). From these results, it was confirmed that changes in the level of measured stiffness can be used to quantify the level of pain felt by a patient.