This study is to examine the effect of robots' language forms on people's acceptance of robots. We applied a concept of social distance to measure people's acceptance of robots. In an experiment, calling participants by name vs. not calling by name as well as the robot's speech styles (familiar vs. honorific), were used to impose a verticality and horizontality of social relationships between participants and robots. After the conversation with a robot, participants rated the robot's interpersonal traits and their comfortable approach distance to the robot, and their response to the robot during the experiment were analyzed. As a result,participants whom the robot called by their name perceived the robot as friendlier. They introduced themselves more actively, and were more intently focused on what the robot said. They asked the robot questions more frequently. Participants called by their names consequently approached the robot more closely than participants who were not called. An interaction effect was found between speech styles and whether names were used in regard to the perceived friendliness of robots, negative response to robots, and comfortable approach distance to robots. We discuss verbal interaction design for increasing people's acceptance of robots. (C) 2012 Elsevier Ltd. All rights reserved.