In this paper, we studied the ultrasonic urine volume sensor of urinary alarm system for home use to detect the time of urination as a assistive methodology for the incontinence patients and the correlation between urine volume and bladder interwall distance by using developed sensor. The developed sensor was designed to minimize the measurement error by using ultrasound with 2.25 MHz center frequency which provides higher resolution as well as longer penetration depth. To verify usefulness of the developed sensor, we performed a preliminary experiment of estimating bladder volume from the measured distance between interior and posterior wall of bladder. In the preliminary experiment, bladder volume estimated from the result using a commercial ultrasonography system. The experimental results show there exists god correlation between the actual urine volume and the measured interwall distance of the bladder. In conclusion, the developed ultrasound bladder volume sensor can be applied to an urine alarm system which provides patient the exact time of urination, it will be contribute in health care and welfare society.