The downsizing of computers enables users to operate computers anywhere. Generally, since a user operates his/her computer by his/her hands, computer operation is unconcealed to surrounding people. On the other hand, there are demands for hidden operation of the computer in situations such as face to face communication and during a meeting. The operation of computer in those scenes often gives a bad impression to surroundings and discommunication. Therefore, in this study, we propose an interface using a user's abdominal circumference as input. The motion of the abdomen is hard to be recognized by surroundings, and a user can move one's abdomen independently of the other body parts. We implemented a prototype of input method using the moving velocity and the absolute size of abdomen as input. Then, we evaluated the granularity, reproducibility, parallelism, resistance to environment, confidentiality and resistance to misrecognition on the proposed interface.