The shepherding problem is to control and guide a flock of multiple autonomous agents by means of one or more external controllable agents. A critical example is a sheepdog/shepherd guiding a flock of sheep/agents and herding them to a target destination. Solving this problem is expected to lead to the development of robots for herding livestock and for guiding people who need evacuation. Strömbom et al. modeled a shepherd's behavior mathematically, in which a single shepherd can herd a flock of agents to a target. They evaluated the performance of the herding algorithm (HA) with a constant difference between the shepherd's and agents' moving speeds. In this study, we evaluate simulated HA proposed by Strömbom et al. for various differences between the shepherd's and agents' moving speeds caused by the agents' stride length and analyze the robustness of this algorithm regarding the agents' moving speeds. The experimental results show that the success of herding is mostly guaranteed when the moving speeds of all the agents in the flock are lower than the shepherd's moving speed. Also, the results show that the herding succeeds even if the flock consists of agents having various moving speeds. From these results, we have clarified that HA with a single shepherd is mostly robust regarding agents' moving speeds.