Although four-wheel steered mobile robots have less turning ability, compared to two-independent wheeled mobile robots, there is an advantage that they are fit for improvement in the movement speed. Note however that since the wheel slippage is increased due to its advantage, four-wheel steered mobile robots cannot apply a popular dead-reckoning method by using rotary encoders. This paper describes a method for tracking a straight line using four-wheel steered mobile robots by applying an image-based control method, which controls such robots without referring to self positions. A fuzzy controller that is one of model-free controllers is adopted, because a relationship between the movement of a robot and the image that is captured by a camera attached on the robot cannot be modeled easily. The proposed method is evaluated through some simulations and real experiments.