In this paper, we examined the effects of the spoken dialogue interface in a dual task situation. One of the well suited situations for the use of the spoken dialogue interface is that including dual tasks, e.g. when several tasks are being carried on at the same time. In order to develop a spoken dialogue system, it is important to consider such a situation and the effects of the spoken dialogue interface in it. It has been said that the merit of spoken dialogue interface is that it doesn't make the user's manual and visual modalities busy. Therefore, it is expected that this interface is effective in a dual task situation. We made experiments to prove this hypothesis. As the result of these experiments, it became clear that spoken dialogue interface was effective in a dual task situation and that considering the situation is important for the design and evaluation of the interface.