We have developed an interactive learning environment for problem-posing of arithmetical word problem as sentence integration that is called MONSAKUN. We have already verified usefulness of MONSAKUN through several classes in a few elementary schools. In this research, to make learning by problem-posing with MONSAKUN more effective for the structural understanding, we have designed an exercise to make the learner engage in self-explanation of their problem-posing. In MONSAKUN, a learner is required to pose a problem by selecting three sentence cards from a set of sentence cards. The cards that are not necessary to pose the problem are called dummy cards. Therefore, the process of problem-posing is regarded as finding process of the dummy cards. Therefore, in this research, the activity of self-explanation of the problem-posing is regarded as the activity to explain a card as a dummy one. Then, the explanation is designed as modification of the original assignment of problem-posing in order to make an assignment that is allowed to use the dummy card to pose a problem. We have developed an interactive environment where a learner is required this modification. In this paper, we report an experimental use of this environment in an elementary school and the results.