Abstract: Blood pressure is estimated in a non-constraining manner continuously and in real time. A biological signal collected from a person's dorsal part is analyzed, vibration in the human body ascribable to a blood flow rate change in a period from the ventricular filling period to the isovolumetric systole is captured, and the state of fluctuation of an index indicating the state of the in vivo vibration is further captured. This fluctuation index correlates with a person's blood pressure. Therefore, it is possible to estimate whether the blood pressure is in a range indicating a normal-range blood pressure or in a range indicating a high blood pressure, only by having the person sit or lie supine on a surface provided with a biological signal measurement device to measure a biological signal in a non-constraining manner, so as to make the biological signal measurement device contiguous with the person's dorsal part.