In this paper, we investigate effects of the flexibility of the body on locomotion of a quadruped robot using computer simulation with physical models. So far, many researchers have used legged robots whose bodies consist of one rigid body and examined dynamical properties, such as stability of gaits and energy efficiency. However, from the observation of animals, it has been suggested that flexibility of the body is important for dynamic locomotion. We used two types of simple physical models, with and without a springy joint in their body to evaluate the importance of the body flexibility on the bounding gait, where left and right legs simultaneously kick the ground. Our simulation results show that when we use the same mechanical energy for these two models, the maximum ground reaction force for the model with a body springy joint is smaller than that for the model without a body springy joint (rigid body) even when their locomotion speeds are identical. This suggests that the flexibility of the body can reduce the foot loading of the robot in the bounding gait.