An arterial-wall stiffness evaluation system comprising: - a cuff to be attached to a part of a living body; - a pressure sensor for detecting pressure in the cuff; - cuff-pressure control means for controlling the pressure in the cuff to be increased or decreased to a predetermined value, based on a value detected by the pressure sensor; and - data processing means for calculating, based on pulse waves detected by the pressure sensor, pulse-wave amplitudes of cuff-pressure pulse waves and blood-pressure pulse waves, and for evaluating an arterial-wall stiffness based on the pulse-wave amplitudes thus calculated, wherein, - from the pulse waves thus detected, a curve is estimated from a set of a plurality of average values, - the arterial-wall stiffness is evaluated by estimating, by numerically integrating the curve, a pressure-diameter characteristic curve, and - the pressure-diameter characteristic curve represents a relationship between a vascular diameter and a transmural pressure applied to a vascular wall, wherein the set of the plurality of average values is obtained by the following steps of: i) obtaining respective ratios between amplitudes of extracted cuff-pressure pulse waves and corresponding pulse pressures in respective pulse-wave zones including a transmural pressure of P applied to a vascular wall and determining an average value of the respective ratios as a value corresponding to the transmural pressure of P applied to a vascular wall, wherein a pulse-wave zone is a zone on an axis representing the transmural pressure which zone corresponds to an amplitude of a blood-pressure pulse wave at a certain time point; and ii) performing the step i) twice or more so as to obtain a plurality of average values each being the average value obtained in the step i). - a cuff to be attached to a part of a living body; - a pressure sensor for detecting pressure in the cuff; - cuff-pressure control means for controlling the pressure in the cuff to be increased or decreased to a predetermined value, based on a value detected by the pressure sensor; and - data processing means for calculating, based on pulse waves detected by the pressure sensor, pulse-wave amplitudes of cuff-pressure pulse waves and blood-pressure pulse waves, and for evaluating an arterial-wall stiffness based on the pulse-wave amplitudes thus calculated, wherein, - from the pulse waves thus detected, a curve is estimated from a set of a plurality of average values, - the arterial-wall stiffness is evaluated by estimating, by numerically integrating the curve, a pressure-diameter characteristic curve, and - the pressure-diameter characteristic curve represents a relationship between a vascular diameter and a transmural pressure applied to a vascular wall, i) obtaining respective ratios between amplitudes of extracted cuff-pressure pulse waves and corresponding pulse pressures in respective pulse-wave zones including a transmural pressure of P applied to a vascular wall and determining an average value of the respective ratios as a value corresponding to the transmural pressure of P applied to a vascular wall, wherein a pulse-wave zone is a zone on an axis representing the transmural pressure which zone corresponds to an amplitude of a blood-pressure pulse wave at a certain time point; and ii) performing the step i) twice or more so as to obtain a plurality of average values each being the average value obtained in the step i).