This paper proposes a basic technology that utilizes natural objects in the environment and uses them as a simple interface. We focus on an interface that uses a small wooden balance beam. Its input is the movement of the user's center of gravity and the balance on the beam. The user controls the interface by hopping onto it, walking back and forth, jumping on it or leaning his body in a direction. Strain sensors are attached to the beam, and by using the sensor data, approximate position of user's center of gravity and change of the balance can be estimated. The interface was applied to an interactive system, which is a combination of a balance beam and computer graphics.