This paper proposes an organ boundary determination method for detecting internal bleeding. Focused assessment with sonography for trauma (FAST) is important for patients who are sent into shock by internal bleeding. However, the FAST has a low sensitivity, approximately 42.7 %, and delays of lifesaving treatment due to internal bleeding being missed have become a serious problem in emergency medical care. This study aims, therefore, to construct an automatic internal bleeding detection robotic system on the basis of ultrasound (US) image processing to improve the sensitivity. Internal bleeding has two key features: it is extracted from low-brightness areas in US images and accumulates between organs. We developed method for extracting low-brightness areas and determining algorithms of organ boundaries by low-brightness set analysis, and we detect internal bleeding by combining these two methods. Experimental results based on clinical US images of internal bleeding between Liver and Kidney showed that proposed algorithms had a sensitivity of 77.8% and specificity of 95.7%.