This research aims to develop a task of geometry proof for improvement of students' abilities to logically analyze and express. With traditional tasks, they were trained to clarify evidence and to logically think and express. In this research, the class was designed concerning the three points below: (1) to have them acquire theorems and the nature of figures, (2) to have them choose the theorem to use on the given condition, and (3) to have them tenaciously work on a task with insufficient condition. They shared their process of thinking by writing and through discussion. In conclusion, they were able to enlarge their knowledge for logical analysis and expression.