We describe artificial subtle expressions (ASEs) as intuitive notification methodology for artifacts' internal states for users. We prepared two types of audio ASEs; one was a flat artificial sound (flat ASE), and the other was a sound that decreased in pitch (decreasing ASE). These two ASEs were played after a robot made a suggestion to the users. Specifically, we expected that the decreasing ASE would inform users of the robot's lower level of confidence about the suggestions. We then conducted a simple experiment to observe whether the participants accepted or rejected the robot's suggestion in terms of the ASEs. The results showed that they accepted the robot's suggestion when the flat ASE was used, whereas they rejected it when the decreasing ASE was used. Therefore, we found that the ASEs succeeded in conveying the robot's internal state to the users accurately and intuitively.