Recently, autonomous robots which operate in unpredictable complex environment, for example in the rescue operation, in space development and so on have attracted much attention. Snake-like robot is one of typical example of them. However, to control the robot in the unpredictable environment is very difficult problem, because to model the environment is impossible and much information should be measured to move the robot at the complex unknown environment. In addition, many computational costs are required to calculate control law for many degrees of freedom. In this paper, for the first step, we consider autonomous control of the snake-like robot which moves to a light source on a horizontal plane that has many unknown obstacles. To solve the problems, we show importance of passive mechanism, and realize an autonomous control for a snake-like robot by utilizing the passive mechanism. To demonstrate the effectiveness of the proposed mechanism, we have developed a snake-like robot and experiments have been carried out. As a result effective behaviors have been realized