A position-detecting device includes a body pressure distribution sensor including a plurality of pressure sensors that outputs pressure signals corresponding to pressure values respectively and are arranged in a matrix pattern, a specific pattern storage unit storing a range that pressure sensors can occupy and a condition of pressure values relating to the pressure sensors within the range as a specific pattern by sleeping posture and body part of a sleeper, a scanning unit applying the specific pattern to body pressure distribution information, which is a set of the pressure values, from a scan starting position to an adjacent position in sequence and to determine whether the condition is satisfied, and a body part position determining unit determining the position of the body part in the sleeping posture corresponding to the position of the specific pattern in the body pressure distribution information when the condition is satisfied.