For accurate insertion by CT-guided robot, we need registration between the coordinate of robot and CT by calculating the section images. In this paper, we intended to develop the geometric marker for registration. The requirements are geometric shapes that the error of 3 DOF (yaw and pitch angles and a craniocaudal distance) between CT image and the marker can be calculated and the material is suitable for CT imaging. Then, marker has four oblique prisms to calculate the error of 3DOF. Oblique prism has advantage on the point of reducing error of angle calculation because of the slope of inverse trigonometric function. From the result of evaluation, accuracy of registration system by geometric marker is satisfactory for insertion to the tumor in lower abdominal region.