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Self–administered questionnaires about physical activity are useful for collecting data to develop public health policies. There is currently no validated physical activity questionnaire, however, for Japanese children and adolescents. The purpose of the present study was to evaluate the validity and reliability of self–administered, physical activity questionnaires for Japanese students.Fifth– and eighth–grade students were asked to complete two questionnaires that estimated the intensity and time of moderate–to–vigorous physical activities in which they participated, and the frequency and duration spent performing sports activities. Students also wore triaxial accelerometers to compare their actual activity levels to their estimates, to investigate the validity and reliability of the questionnaires.The intensity and time spent performing moderate–to–vigorous physical activity that were estimated from the questionnaire were higher than those measured by accelerometry (9–161% of accelerometry). Questionnaire data were highly correlated with accelerometer data for eighth graders (Spearman correlation, 0.642–0.754), but the correlations were lower for fifth graders (≤0.331). Furthermore, there was higher repeatability in the data collected from eighth graders (intraclass correlation, 0.625–0.645) than from fifth graders (0.136–0.194).Questionnaires may be useful in epidemiological studies for ranking physical activity levels of adolescents, such as a confounding factor for other lifestyle surveys, but these surveys are less accurate for younger children.