This study intends to explore the means for improving math classes using open tasks by reflecting on the experiences of teachers in planning and implementing open classes. Toward this end, the study reviewed the open-ended approach in Becker and Shimada(2004), the analysis framework of solution spaces in Kim, Nam-gyun et al.(2022), and five practices of planning and implementing math classes in Smith and Stein(2013). Against this background, four teachers planned and reflected on open classes based on analysis of the solution spaces out of which two teachers in charge of fifth and sixth graders each conducted instruction for 25 students per class. Afterward, analysis of the solution spaces to an open task served as the basis for the meaningful mathematical discussions of the teachers in the open class. The study examined the difficulties and challenges faced by the teachers in this process. In addition, the study presented suggestion on improving open classes while reflecting on this result. As such, the implementation process of this study is expected to be used by teachers who want to plan and implement open classes.