In this paper, we derive a dynamic model and a trajectory tracking control law for snake robots, which have passive wheels and active joints. We remove passive wheels of some links, and introduce shape controllable points in the snake robot's body. For a trajectory tracking control of snake robots, it is one of the important problem to avoid the singular configuration like a straight line. Using input-ouput linearization for only the head position of snake robots based on dynamic model, we can control the head position by PD feedback, but the robot will converge to the singular configuration. In order to solve this problem, we propose a control law for shape controllable points, based on an index related to the singular configuration. Using the proposed control law, the robot can accomplish trajectory tracking of the head position without converging to the singular configuration. Simulation results show the effectiveness of the proposed control law.