In the field of computer aided surgery, augmented reality (AR) technology has been successfully used for enhancing accuracy of surgery and making surgeons convenient by visually assisting them in performing a number of complicated and time-consuming medical operations. However, there are still medical operations that do not receive the benefit of AR technology. As a representative one, surgeons still use an ink pen when they mark surgical targets for scheduling an operation. The ink pen is inconvenient because the mark drawn by the foreign matter is not easily modified or deleted. And the ink pen is also unlikely to be sanitary. In this paper, we propose an interactive user interface based on direct-projected augmented reality (DirectAR) technology for handling all these problems with the ink pen and its validity is shown in experimental results.