A biological body state estimation device for estimating a state of a human being by using a biological signal sampled from the upper body of a human by biological signal measuring means, wherein the device comprises: first frequency calculating means for acquiring a zero-crossing point at which a time-series waveform of a biological signal obtained by the biological signal measuring means is changed from positive to negative and for acquiring the time-series waveform of the frequency of the biological signal by using this zero-crossing point; first frequency slope time-series analysis calculating means for performing movement calculation for acquiring a slope of the frequency at each predetermined time window set with a predetermined overlap time in the time-series waveform of the frequency of the biological signal obtained by the first frequency calculating means and for outputting a time-series change of the slope of the frequency obtained at each time window as a frequency slope time-series waveform; power spectrum calculating means for applying frequency analysis to the frequency slope time-series waveform obtained by the first frequency slope time-series analysis calculating means and acquiring a power spectrum of each frequency corresponding to a functional adjustment signal, a fatigue reception signal, and an activity adjustment signal determined in advance; and determining means for determining a state of a human being by acquiring a time-series change of the power spectrum of each frequency corresponding to the functional adjustment signal, the fatigue reception signal, and the activity adjustment signal acquired by the power spectrum calculating means, and determining the state of the human being on the basis of how the power spectrum of each signal changes as time elapses. first frequency calculating means for acquiring a zero-crossing point at which a time-series waveform of a biological signal obtained by the biological signal measuring means is changed from positive to negative and for acquiring the time-series waveform of the frequency of the biological signal by using this zero-crossing point; first frequency slope time-series analysis calculating means for performing movement calculation for acquiring a slope of the frequency at each predetermined time window set with a predetermined overlap time in the time-series waveform of the frequency of the biological signal obtained by the first frequency calculating means and for outputting a time-series change of the slope of the frequency obtained at each time window as a frequency slope time-series waveform; power spectrum calculating means for applying frequency analysis to the frequency slope time-series waveform obtained by the first frequency slope time-series analysis calculating means and acquiring a power spectrum of each frequency corresponding to a functional adjustment signal, a fatigue reception signal, and an activity adjustment signal determined in advance; and determining means for determining a state of a human being by acquiring a time-series change of the power spectrum of each frequency corresponding to the functional adjustment signal, the fatigue reception signal, and the activity adjustment signal acquired by the power spectrum calculating means, and determining the state of the human being on the basis of how the power spectrum of each signal changes as time elapses.