For the care of people in bed, the assistance or support for their body motion is an essential task. In the absence of helpers or at night, however, preventive measures are being required to avoid sudden accidents such as a fracture of bone due to a fall from a bed or suffocation due to turning in bed. In this paper, a new method was proposed for monitoring their body motion, which is able to supply a 24-hour monitoring with a low cost and high accuracy. Being different from commonly used methods in which three-dimensional coordinates for the markers attached to the body are obtained from sequential image processing, this proposed method employs two-dimensional positioning detectors to determine the three-dimensional coordinates of the markers via a perspective view transformation. A prototype system was developed based on our ideas, and its measurement evaluation was conducted for a fixed marker and a repetitive circular motion marker. The results showed that positions obtained 1% for the fixed marker vary within and reproduce with an error of ±3.6 mm for the repetitive circular motion marker with a swing of ±100 mm. An application of the system to a subject turning in bed demonstrated the system's feasibility for body motion monitoring from the motion tracks together with rolling angles and the breathing of a subject.