Human cognitive mechanisms have been studied for the design of user-friendly interfaces. One of the key issues is a sense of agency, which is defined as the sense that “I am the one who is causing this action.” The user interface is important; it can alter the feeling of sense of agency. In this research, we focus on a prime stimulus and evaluate the effect thereof by experiments with participants. A ball moves in a circle on a monitor at a constant speed and participants stop it by pushing a key. They were given both prime stimulus and feedback stimulus and indicated if they were the agent who stopped the ball, i.e., they felt a sense of agency. From the results of the experiment, we found that the prime stimulus can have both a positive and negative influence on the sense of agency when human prediction is unreliable.