This study investigated the effect of speech errors made by a robot on people’s perception of the familiarity of the robot. Four types of speech errors were implemented for a communicating robot. Subjects’ perception of the familiarity of the robot, along with other subjective evaluations of impressions of the robot, were measured in an experiment and compared between conditions with and without speech error. Results showed that speech error improved the familiarity score when the robot did not make any speech errors in the first contact with subjects and then made speech errors in the second contact on a separate day. On the other hand, speech errors lowered people’s perception of the sincerity of the robot. These results imply that the robot should not make speech errors in the early stage of engagement with human users, while some speech errors after the users become accustomed with the robot might be effective in improving users’ perception of the familiarity of the robot. Results also showed that speech error improved familiarity for people having low-scoring attitudes about the diversity and operation of robots.