The purpose of this study is to estimate the sensitivity of an infrared ray projection type three-dimensional shape measurement device (Microsoft Kinect), aiming at realtime monitoring of the position of a patient. Kinect is a motion sensing input device developed by Microsoft for the Xbox 360 video game console. We used a rectangular box of 465 mm x 315 mm x 160 mm as the object simulating an adult male's chest to evaluate the sensitivity of measuring the position of the object by a Kinect sensor system. We used the tilting and rotating mechanical stage with rotating angle accuracy of 0.1 degree to move a measuring object. We used an index Δp(t) which is calculated by summing up the square of difference of the distance between the initial and final positions over the whole surface area of the measuring object. We have developed a computer program using OpenCV and OpenNI to calculate the index Δp(t) to estimate the movement of the measuring object. It was shown that the minimum detectable movement of the object was 1 mm in shift and 1 degree in rotation, respectively. From the measurements, it was suggested that this system can be applicable to real-time monitoring in a variety of medical fields such as radiotherapy and temporal subtraction radiography.