This paper proposes a sensing system for detecting the accident of the bed fence. The authors inspect an application of the ultrasonic wave sensor which does not affect the human body. Recently，many elderly person accidents happen frequently in the bed. Especially，there are a lot of fatal accidents that relate to the bed fence. The number of accidents from which the elderly is placed between the fence and fence of a bed has increased rapidly. To detect such accidents，the care of the elderly person by the caretakers is necessary. However，such care for a long time becomes a big burden for them. Moreover，there is no existing research related to the bed fence accidents now. Therefore，we propose a detection system，with which an elderly person is monitored and the caretakers is alarmed of accident.