Few studies in Japan have investigated smartphone addiction utilizing an international scale. This study aimed to evaluate the reliability and validity of the Japanese version of the Smartphone Addiction Scale (SAS) in Japanese youths and to examine the factors underlying smartphone overuse. Methods: We conducted a questionnaire survey in 2018 with participants from one high school (n=1,050) and one vocational school (n=83; age range, 15-24 years; median age, 17 years) in Japan. Data from 1,037

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... (male: 60.8%, n=631) questionnaires were analyzed. Results: Factor analysis showed six factors with factor loading ranging from 0.42 to 0.72 for the Japanese version of the SAS. Cronbach's alpha was 0.94 for the 33 items in this scale. The scale showed good reliability for test-retest scores ( intraclass correlation coefficient 3,1 =0.92). Multiple linear regression analyses showed that the factors related to smartphone overuse were female gender, long smartphone usage duration, poor mental health, contact with virtual friends, and being a smartphone zombie. Contact with family and real friends were not associated with smartphone overuse. Conclusions: Although smartphone overuse differs depending on one's cultural and social background, the Japanese version of the SAS is as good as the other language versions. Thus, this scale was shown to be reliable and valid in the Japanese youth.