A sleep state of a subject is estimated from respiration signals that are readily detected without restraining the subject. From a voltage waveform based on respiratory movement of a person, positive peak values and the (time) interval between adjacent peaks are calculated. Also obtained is the area of a region that is enclosed by the voltage waveform and a time axis between peaks, and the average value and dispersion value of the obtained inter-peak areas are calculated. The calculated values are used to set a preliminary sleep state value for a given period, and sleep state values over plural periods are factored in to estimate a sleep state at a given time point.