The present invention makes it possible, by simply acquiring images of a test subject's skin multiple times, to estimate the circadian rhythm of the test subject from said multiple images without requiring genetic measurement, cell culture, frequent blood collection, analysis at a special research facility, etc. by being provided with: an image-acquiring unit (122) for acquiring multiple images of the skin of a test subject taken over multiple time points by an imaging device (104); a body hair-specifying unit (123) for specifying growth phase body hair of the test subject from the images for each of the multiple images acquired by the image-acquiring unit (122); and a circadian rhythm-estimating unit (124) for estimating the test subject's circadian rhythm on the basis of temporal changes in the length of the growth phase body hair determined as a result of the body hair-specifying unit (123) specifying growth phase body hair from each of the multiple images.
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