The present invention uses a time-series waveform of biosignal information collected from the back of a person and including biological sound and vibration (back body surface pulse wave). The back body surface pulse wave is pressure vibration arising from cardiac and aortic motions, and includes ventricular systolic and diastolic information and information on vascular wall elasticity which serves as an auxiliary pump for the circulation, and is considered as a vibration system including both the damping of viscous damping friction and the damping of solid friction. The back body surface pulse wave fluctuates according to a flow rate (stroke volume) of blood pumped out from the heart, and the fluctuation of this flow rate is reflected in the amplitude of the time-series waveform of the back body surface pulse wave. Therefore, by applying a solution which calculates a damping ratio of a free damped vibration waveform to compare predetermined waveform components in one cardiac cycle, preferably, amplitudes of two waveform components in an amplification phase, it is possible to detect a biological state, in particular, a biological state associated with a blood pressure.
