We are motivated to deal with a biped robot by an existence of almighty powerful control mechanism of human. Although a visual information, in particular, plays important role in order to realize admirable intelligence in a humanoid robot, we must assert a vision to be one of the developing sensors. In fact, there are many difficulties of both aspects of restrictive hardware resources for a vision and real-time image processing for motor control. In this paper, we clarify a structure and a nature of a visual information processing system which should be equipped with a biped robot. For this purpose, a human eye structure and its movements, and brain motor control through a central nervous system are considered as an analogy with robotics. We shall show that a gaze control mechanism like human is absolutely indispensable for a vision system of a biped robot. Furthermore, we investigate considerable differences between the human eyes and the biped robot ones. This allows us to propose a new, simple and intuitively understandable criteria, called stably gazing scope, for evaluating gaze control performance in the sense of availability of visual information for robot motion control.