This study aims to investigate effects of lessons to make children become aware of fiction and its merits by actively showing them principles of fiction in a unit or one lesson in four areas of mathematics such as number and calculation, quantity and measurement, figures, and relationships of numerical quantity on cultivation of children's attitudes toward "concepts of fiction" to use it actively and voluntarily. Results show that children are more likely to develop their understandings of concepts of fiction by intentionally dealing with fiction in class. Additionally, contents of teaching for each grade and acquired attitudes by children in one unit are clear when lessons are carried out on the basis of hypothetically thinking about how children raise their attitudes corresponding to each grade. Implications of results are discussed to propose effects of lessons to make children foster "concepts of fiction"