A body motion sensor, such as an SAS diagnostic device, with which a subject can be examined in a state close to that in an ordinary living environment and it is possible to clearly discriminate between an apnea state and a hypopnea state by means of a relatively simple analytical method and an optical fiber sheet suitable for use in the sensor. The body motion sensor, such as an SAS diagnostic device, comprises a light source (2) such as an LED, an optical fiber sheet (10), and an analysis unit (3) that determines fluctuations in the quantity of light outputted from the optical fiber sheet (10) and detects motions of the body. As the optical fiber sheet (10), use is made of an optical fiber sheet (10) comprising a planar object (20), such as a fabric, paper, or plastic sheet, and graded-index optical quartz fibers (11) directly or indirectly disposed thereon or affixed thereto When the sheet (10) is produced, it is preferred to use a pressure-sensitive adhesive sheet (30) comprising a sheet support (31), such as paper or fabric, and a pressure-sensitive adhesive layer (32) formed thereon.
