We describe artificial subtle expressions (ASEs) as an 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 robots lower level of confidence in its suggestion. We then conducted a simple experiment to observe whether the participants accepted or rejected the robots suggestion based on the ASEs. The results showed that they accepted the robots suggestion when the flat ASE was used, whereas they rejected it when the decreasing ASE was used. We thereby concluded that the ASEs succeeded in conveying the robots internal state to users accurately and intuitively.