<p>The invention addresses the problem of detecting menstruation easily, inexpensively, highly reliably, and minimally invasively. A solution to the problem is a biorhythm-detecting system characterized in being provided with: a body movement sensor, which acquires sensor data representing movements of the body; a target period-calculating unit that calculates a period to be analyzed from the sensor data; a main cyclic component-detecting unit that tabulates sensor data and detects the main cyclic component for each specified period within the period being analyzed; a representative value-calculating unit that calculates a representative value for the main cyclic component from the group of multiple main cyclic components calculated within the target period; and a biorhythm-detecting unit that detects the biorhythm cycle from the calculated representative value for the main cyclic component.</p>
