Abstract – Violating the privacy of individuals with sys- tems that measure daily human activities is unacceptable in oursociety. Inthispaper,theauthorspropose asystemusing ultrasonic radar embedded in the ceiling of a room to locate human individuals with a minimum violation of privacy. It is expected that thissystemwill be used inthe preventionof ac- cidents among aged persons suffering from Alzheimer’s dis- ease. The proposed system determines the three-dimensional positionofa person’sheadbyassumingthatthehumanhead is an object that will move at a relatively high vertical po- sition within a living area. In this study, the method used to locate the human head is analyzed theoretically from the perspective of acoustics. The feasibility of the proposed sys- tem is confirmed by experimental results using a system con- structed by the authors.