Geometry is an important part of mathematics, and recently spatial ability is emphasized as one of many objects through geometry instruction in the objects of mathematics education. A lot of studies showing the relationship between spatial ability and mathematics achievement, and studies related to gender differences in spatial ability are being continued. Many educators of mathematics assert the extension of student's spatial ability through geometry instruction. But, there are few studies on the construction of geometry section in mathematics textbook with respect to extension of spatial ability. This study investigated the relationship between spatial visualization ability-a particular subset of spatial ability-and mathematics achievement. It investigated also the existence of extension of spatial visualization ability as ascending grades, so this study presents whether the geometry section in mathematics textbook is constructed adequately to that object. Two opposite type middle schools were sampled in this study: all-boys school, and all-girls school. This research got a Purdue Spatial Visualization-Rotations Test to all grades of the sample middle schools, and gathered mathematics scores of National Achievement Assessment tested last year for second and third grades (middle school) students. The results of this research as follows: First, there exist a positive relationship between spatial visualization ability and mathematics achievement in the middle school students. Second, there does not exist grades differences in spatial visualization ability of middle school students except for the difference between first - second grades of boy students, i. e., their spatial visualization ability does not be extended as ascending grades. Thus, the hypothesis was taken that the geometry section in mathematics textbook was one of the cause of no extension of spatial visualization ability. And for the geometry section in first and second middle school mathematics textbook, the content construction was analyzed. As a result, the part related to spatial visualization ability in the geometry section is about 30.3% in first grade, and about 11.8% in second grade. This can be related the relative difference between the difference of average of 1-2grades(increasing 1.09) and the difference of average of 2-3grades(decreasing 0.02), in comparison with the grades difference of spatial visualization ability.