If a dialog system can respond to the user as reasonably as a human, the interaction will become smoother. Timing of the response such as back-channels and turn-taking plays an important role in such a smooth dialog as in human-human interaction. We developed a response timing generator for such a dialog system. This generator uses a decision tree to detect the timing based on the features coming from some prosodic and linguistic information. The timing generator decides the action of the system at every 100 ms during the user's pause. In this paper, we describe a robust spoken dialog system using the timing generator. Subjective evaluation proved that almost all of the subjects experienced a friendly feeling from the system.