The adolescent is the big turning point during the life. Adolescents must find positive answer for the question "what is with the quality of oneself" through interactions with neighboring people to develop SELF-IDENTITY. On the other hand, Social Network Services (SNS) are highly familiar to them, making "SNS addiction" become a big social problem. This study attempted causal relationship among self-identity development and using SNS, consequently SNS addiction, from the viewpoint of personal connections. A two-wave panel survey was conducted in April and July 2013. One hundred and thirty two Undergraduate students were requested to complete scales, which measures Identity development, loneliness, SNS addiction, the number of acquaintance in real life connected on SNS (real network size) and connected only on SNS (net-only network size). Results indicated that real network size significantly caused directly Identity development while net-only network size did not. Moreover, for those who had huge real network size, Identity development was promoted as net-only network size was huge. These results suggest that only for those who have the abundant network in real life, it may be said that the relations only on SNS become meaningful for developing their own Identity. Furthermore, they can show knowledge of importance to cultivate network in real life at first to receive benefits of using SNS.