In order to continuously estimate blood pressure in a non-constraining manner and in real time, the present invention is configured to analyze a biological signal collected from a person's dorsal part, captures vibrations in the body (in vivo vibrations) generated by changes in blood flow between the ventricular filling period and the isovolumetric contraction period, and further captures fluctuations in an index (index relating to fluctuations [fluctuation index]) indicating the state of the in vivo vibrations. The fluctuation index is associated with information relating to the person's condition, in particular the person's blood pressure, as described above. On the above basis, the present invention makes it possible to easily estimate a person's blood pressure, and in particular, whether the blood pressure is within a normal blood pressure range (diastolic blood pressure of less than 90 mmHg, systolic blood pressure of less than 140 mmHg) or in a high blood pressure range, by simply having the person sit or lie supine on a biological signal device that can measure a biological signal in a non-constraining manner and is attached to a vehicle seat, a chair for home, office or other uses, sleeping equipment such as a bed, or the like so that the biological signal device is contiguous with the person's dorsal part.
