A pronation and supination of forearms is used as one of diagnostic indices for developmental disorders. However, this index has a demerit in which evaluation results between doctors are not consistent. The present study aims to establish more quantitative and evaluating method. We have developed a simple and portable evaluation system to measure pronation and supination of forearms. In this study, we have measured the pronation and supination of the forearms of 261 subjects aged 7 to 12 years (healthy children : 201, ADHD children : 60) and compared the function of healthy children's pronation and supination with the function of ADHD children's pronation and supination by new indices. From the results, we could obtain a difference between healthy children and ADHD children. The results showed that our developed system has the potential to become diagnostic criteria for developmental disorders.