During a period of servo control, an artery volume of a peripheral site is detected by an artery volume sensor arranged at a peripheral site (site on the peripheral side than a measurement site) of a person to be measured. If an amount of change in the artery volume of the peripheral site or a value of the artery volume is greater than or equal to a predetermined ratio of a reference value at the beginning of the measurement, the servo control and a blood pressure determination process are continued. When detected thereafter that the amount of change in the artery volume of the peripheral site or the value of the artery volume is smaller than the predetermined ratio of the reference value at the beginning of the measurement, the measurement is stopped at the relevant time point.