When adults talk to infants, they tend to use infant-directed speech (IDS) rather than adult-directed speech (ADS). IDS is considered to draw the infants’ attention more than ADS, to convey adults’ emotional states to infants more easily, and to make language acquisition easier for infants. It is not clear, however, whether the use of IDS has some effects on the adults as well as on the infants. In this research, we focused on one of the most distinctive features of IDS, a high-pitched voice, and conducted two human-robot interaction experiments to examine whether or not the use of a high-pitched voice by the participants him/herself can create a good impression of the robot in their minds. The results showed that the participants in our experiments seemed to be more favorably impressed with the robot when they used a high-pitched voice.