A bed-leaving sensor for detecting bed-leaving by a user on a bed, including: a sitting position detection member that detects a sitting position of the user based on a detection value of a pressure sensor arranged on the bed and a bed-leaving behavior detection member that detects bed-leaving behavior of the user with at least one of the following as a condition: (i) the sitting position being detected within a preset bed-leaving expectation region, (ii) a movement volume of a sitting position center of gravity which is a center of gravity of the sitting position exceeding a given threshold value within a given time, and (iii) the sitting position center of gravity moving toward a preset bed-leaving range.
