An in-bed state monitoring system (100) for monitoring an in-bed state of a subject on a bed includes: four load detectors (11, 12, 13, 14) which are configured to be placed at four corners of the bed to detect a load of the subject； and an edge sitting posture determining unit (33) configured to predict that the subject is reaching a state of edge sitting posture based on a change in a magnitude relation between detection values from two load detectors, which are of the four load detectors and are placed to interpose the bed between the two load detectors in a diagonal direction of the bed.
