This study examines whether social activities with parents, online and offline social self-efficacy, and attitudes toward gaming are associated with the degree of game addiction among adolescents. Using data from a survey of 600 middle- and high-school students in South Korea, we tested the relationships of personal characteristics (grade point average and time spent on gaming each day), social self-efficacy (both on- and offline), general social activities (with parents, friends, and teachers), gaming activities with parents, and attitudes toward gaming (those of self, parents, friends, and teachers) with the degree of game addiction. In addition, we conducted ANOVA tests to determine the differences among three groups: non-addicts (NA), possible (mild or moderate) addicts (PA), and Internet addicts (IA). The results show that social self-efficacy in the real world (offline) was negatively related with the degree of game addiction, whereas social self-efficacy in the virtual world (online) indicated a positive association. Social activities with parents are negatively associated with game addiction, although no relationship is found between gaming activities with parents and game addiction. Parental attitude toward gaming has a negative relationship with the addiction. Results and implications are discussed.