To assess the changes in attitudes toward smoking after the smoking-prevention classes presented by Kumamoto Tobacco-Free Forum, this study administered a questionnaire survey to first-year junior high school students before and after a smoking-prevention class at a junior high school. The questionnaire that was used to pre and post comparison comprised thirteen items, ten from the Kano Test for Social Nicotine Dependence (KTSND-Youth) and three as follows: “I think I will be a smoker in the future,” “I think I will smoke about once,” and “What do you think of people around you who smoke tobacco?” The results of this pre- and post-class questionnaire indicate that attitudes toward seven items improved in a statistically significant way. These included four from KTSND-Youth: “Smokers cannot stop even if they want to,” “Smoking is adult-like and cool,” “Smoking is enjoys taste and a fragrance,” and “If it’s a place with an ashtray, it’s okay to smoke.” The smoking-prevention classes significantly decreased total KTSND-Youth scores. Furthermore, the analysis has clarified that the KTSND-Youth items: “Smoking is adult-like and cool” and “Doctor and school teachers say, ‘Tobacco is no good too much”, as well as the item “What do you think of people around you who smoke tobacco?” are significantly influencing factors for the item “I will be a smoker in the future.” These results indicate that the smoking-prevention classes presented by Kumamoto Tobacco-Free Forum were effective for decline in social nicotine dependence of the first-year students in junior high school.