People disclose their personal information related to daily lives on social networking sites(SNS) using tests, pictures, and video clips. This phenomenon is being accelerated by the wide spread of smartphones. By doing so, people attempt to build and maintain very close relationships with others such as family members, friends, and acquaintances. Despite the fact that such self-disclosure contributes to building social relationships, it brings about privacy concerns. This is because personal information posted on SNS can spread online through social networks and can be easily abused. Although many people have the worry about an invasion of privacy, they continue to disclosing their personal information on SNS. Therefore, this study explores self-disclosure on SNS, applying the theory of planned beahvior. Moreover, this study sheds light on key factors forming attitudes towards self-disclosure on SNS.