We describe an ``adaptation gap'' that indicates the differences between the functions of artificial agents users expect before starting their interactions and the functions they perceive after the interactions. We investigated the effects of this adaptation gap on users' impressions of the artificial agents because any variations in impressions before and after the start of an interaction determine whether the user feels that this agent is worth continuing an interaction. The results showed that the positive or negative signs of the adaptation gap and the subjective impression scores of the agents before the experiment affected the final users' impressions of the agents significantly.