The sleeping-posture-control bed system includes a bed floor including a back-lifting portion for lifting the users back, a bed-floor support body supporting the bed floor, and a back-lifting driver for lifting the head side of the back-lifting portion. The back-lifting portion includes a back-receiving surface and a head-receiving surface tiltable at different angles. The back-lifting driver lifts the back-receiving surface and the head-receiving surface, satisfying Mathematical Formula (1): 0°<&thetasx&nlE70°, &minus45°&nlE&thetasy<0°, and &minus30°&nlE&thetasx+&thetasy &emsp&emsp(1) where &thetasx is the tilt angle of the back-receiving surface when tilted, and the tilt angle of the back-receiving surface when the back-receiving surface is not lifted is set to be 0° and &thetasy is the tilt angle of the head-receiving surface when the angle of an extended line of the back-receiving surface at the tilt angle &thetasx is referred to as 0°.
