The purpose of this study was to discuss about application of functional assessment in early childhood setting, through a case of preschooler with pervasive developmental disorder who was crying at preschool activities frequently. By using functional assessment, the situation of crying could be summarized in some reasons (scare of the mother and child separation / can't see her way / difficulty of the activity / difficulty of a position to sit) and the supports that based on the every reasons could be done. As a result, crying of the child decreased gradually and she could participate in preschool activity meaningfully. It was thought that it was necessary to examine about (1) the consideration of the variety of the preschool's activity, (2) the combination with the method to be able to involve plural teachers, (3) the examination of the method to facilitate the process of the support, in future.