The implementation of learning by problem posing in practical use is facing the issue of inefficiency in time needed for assessment and giving feedback to students posed problems.As a solution of this problem,we have developed tablet PC-based software for learning by posing arithmetical word problems named Monsakun.The software is based on Triplet Structure Model of arithmetical word problem.In this research,we investigated elementary school students performance in Monsakun.The result shows that students did not pose problems randomly and with some sort of thinking.With further analysis of constraints,we also found that the frequent errors contain few non-meaningful errors and students tried to pose problems satisfying as many constraints required in each assignment as possible.The process of understanding assignment requirements and relating them to suitable sentence cards is an important point especially for young learners to reach deep understanding of the structure of arithmetical word problems.