A task instruction is carried out in various situations such as a cooking training. In the task instruction, a lot of information is translated by interactions between an instructor and a learner. So, the task instruction requires that the instructor and the learner to be at the same place and time. If we use a robot for mediating between them, such limitations could be eliminated. In this report, we describe a model of interaction between robot and person. In order to better understand the task, the robot plans and executes appropriate interactions such as questions to person according to the current model of the person’s task understanding.