Speech recognition errors are inevitable in a speech dialog system. It is important to provide a dialog flow that allows the user to correct system errors and quickly return to the original dialog. This paper describes explicit, final and implicit confirmation styles that are implemented in the CAMMIA speech dialog system, and compares them from the viewpoint of usability. Our results show that a final confirmation with fewer confirmation turns is preferred by the user when there is no error in the dialog. On the other hand, an explicit confirmation is preferred when an error occurs.