A biological state estimating apparatus comprising: a pair of electrocardiographic electrodes for detecting an electrocardiogram signal; a photoplethysmographic sensor for detecting a photoplethysmogram signal, the photoplethysmographic sensor having a light-emitting element and a light-receiving element ; peak detecting means configured to detect a peak of an electrocardiogram signal detected by the pair of electrocardiographic electrodes , and a peak of a photoplethysmogram signal detected by the photoplethysmographic sensor ; pulse transmission time calculating means configured to calculate a pulse transmission time from a time difference between the peak of the photoplethysmogram signal and the peak of the electrocardiogram signal which are detected by the peak detecting means ; pulse transmission time storing means configured to store the pulse transmission time calculated by the pulse transmission time calculating means ; correlation information storing means storing sleep state correlation information determined in advance on a basis of a relationship between pulse transmission time before sleep, pulse transmission time after sleep, and state of sleep; and biological state estimating means configured to estimate a quality of sleep indicative of a degree of recovery from fatigue of a user, on the basis of a first pulse transmission time before sleep calculated by the pulse transmission time calculating means and stored in the pulse transmission time storing means, a second pulse transmission time after sleep calculated by the pulse transmission time calculating means , and the sleep state correlation information. a pair of electrocardiographic electrodes for detecting an electrocardiogram signal; a photoplethysmographic sensor for detecting a photoplethysmogram signal, the photoplethysmographic sensor having a light-emitting element and a light-receiving element ; peak detecting means configured to detect a peak of an electrocardiogram signal detected by the pair of electrocardiographic electrodes , and a peak of a photoplethysmogram signal detected by the photoplethysmographic sensor ; pulse transmission time calculating means configured to calculate a pulse transmission time from a time difference between the peak of the photoplethysmogram signal and the peak of the electrocardiogram signal which are detected by the peak detecting means ; pulse transmission time storing means configured to store the pulse transmission time calculated by the pulse transmission time calculating means ; correlation information storing means storing sleep state correlation information determined in advance on a basis of a relationship between pulse transmission time before sleep, pulse transmission time after sleep, and state of sleep; and biological state estimating means configured to estimate a quality of sleep indicative of a degree of recovery from fatigue of a user, on the basis of a first pulse transmission time before sleep calculated by the pulse transmission time calculating means and stored in the pulse transmission time storing means, a second pulse transmission time after sleep calculated by the pulse transmission time calculating means , and the sleep state correlation information. <EOS>