In assessing a speech dialog system, changes in the user's responses to exposure to the system must be considered. In this paper, the changes in user's responses with the repeated use of a speech dialog system are described focusing on different prompt styles. Experiments examine four different styles of prompts to compare their effectiveness. In the experiments, the dialogs between a user and the system are based on the Wizard-of-Oz method. The result shows that at least two back to back trials are necessary to achieve a stable response. This paper describes the experiments and its results.