In this study, we propose an instructional method to develop "the critical thinking in science" for junior high school students. In the method students will sketch "causal relation map" which illustrates reasoning processes of causal relations. The maps are examined by students to make own reasoning process explicit. In order to investigate effectiveness of the method, we conducted a class in which 81 students take a science course at an affiliated junior high school. Based on their writings on questionnaires and worksheets for the class, we conclude that our method can improve their circumspection which is a part of the critical thinking attitude of science.