For a mobile robot to serve people in actual environments, such as a living room or a party room, it must be easy to control because some users might not even be capable of operating a computer keyboard. For non-expert users, speech recognition is one of the most effective communication tools when it comes to a hands-free (human-robot) interface. This paper describes a new mobile robot with hands-free speech recognition. For a hands-free speech interface, it is important to detect commands for a robot in spontaneous utterances. Our system can understand whether user's utterances are commands for the robot or not, where commands are discriminated from human-human conversations by acoustic features. Then the robot can move according to the user's voice (command). In order to capture the user's voice only, a robust voice detection system with AdaBoost is also described.