A dialog system is provided that enables appropriate processing of reinput by a user without describing a dialog scenario on the assumption of reinput by the user. The dialog system includes a reinput acceptable state storage unit that stores a state prior to transition and a reinput recognition candidates set storage unit that stores a recognition candidates set assigned to the state prior to the transition. At the time of a user's input, both of the recognition candidates set associated with that state and the recognition candidates set stored in the reinput recognition candidates set storage unit are referred to so as to determine a recognition candidate. In the case where the determined recognition candidate is a recognition candidate described in the recognition candidates set stored in the reinput recognition candidates set storage unit, the state stored in the reinput acceptable state storage unit is determined as the state to be changed.