The purpose of this paper is to analyze the educational effects of class size and team teaching on achievement, based on a national pupil and student survey in Japanese primary and junior high schools. The smaller the class size is, the better the quality of the class in regards to points of students' animation, concentration and pleasure, which leads students to higher scores of achievement. The educational effects of team teaching is improved individual guidance, especially for students of larger classes in primary schools and for law achievers in junior high schools. On the whole, however, students do not evaluate team teaching highly. To acquire a sound educational foundation, it is obviously advisable to reduce class sizes and to make better use of team teaching from the point of close guidance.