This paper reports on the practice of a lesson in a junior high school in which Error-based Simulation (EBS) was used to help students learn elementary mechanics. EBS simulates the motion of a mechanical system assuming that students' erroneous solutions were correct. EBS has been proved to be effective in laboratory experiments. Twenty-four students participated in the lesson lasting 110 minutes and learned mechanics by using the system which generates and shows EBS. They also worked on the pre-/ post-/delay-tests and were interviewed by the teacher after the lesson. The results are as follows: (1) Through the highly motivating lesson activities, the students discovered the knowledge of mechanics by themselves, (2) It was highly effective in improving the students' performance in solving mechanics problems. Furthermore, this improvement was maintained, and (3) the usability of the system proved sufficient for practice in classroom lessons. These results arc encouraging for the practical use of EBS in junior high school lessons.