This paper proposes a monitoring system which is able to detect the elderly person's abnormalities in a bathtub. Recently in Japan, the number of the elderly people died in accidents while taking a bathtub, especially the apparent drowning accidents, has been increased. According to the Ministry of Health vital statistics, the elderly person of 65 over the age accounted for more than 80 percent of the total. Cause of such accidents is a disturbance of consciousness caused by fluctuations in blood pressure while taking a bathtub. In this context, it is important that the system can detect elderly person's abnormalities in a bathtub. We inspect an application of the ultrasonic wave sensor which does not affect the human body. In addition to the function of preventing drowning, the system can detect bather's fall in a bathtub. By using this system, we will be able to find accidents immediately.