This study examined the effects of consumers' product knowledge and additional information on attitudes toward extended brands in the context of brand extension. Specifically, the research investigated what type of additional information is needed for consumers' knowledge of the parent brand to favorably improve attitudes toward the extended brand product. For this investigation, consumers' knowledge levels of the parent brand product were classified as novice and expert; extended products were categorized as function-oriented products and prestige-oriented products; and the types of additionally provided information were divided into specific attribute information and abstract benefit information. Here, function-oriented products refer to products evaluated primarily on the basis of their specific functional characteristics, while prestige-oriented products refer to products with a concept signifying consumer self-expression. The key findings are as follows. First, consistent with prior research, novices tended to accept extended products utilizing the parent brand more favorably than experts. This phenomenon was limited to cases where there was functional concept consistency between the parent brand product and the extended brand product. When symbolic concept consistency existed, no difference between novices and experts was observed. Second, the same phenomenon appeared when additional information was provided to emphasize the similarity between the parent brand product and the extended brand product. When functional concept consistency existed, the addition of specific attribute information aided brand extension, while when symbolic concept consistency existed, the addition of abstract benefit information helped form favorable attitudes toward the extended brand. Therefore, even when additional information is provided, concept consistency between the parent brand and the extended brand must be carefully considered. Third, concept consistency and consumers' knowledge levels showed an interaction effect. When functional concept consistency existed, attribute information primarily influenced attitude change, and its effect was greater for novices than for experts. Conversely, when symbolic concept consistency existed, abstract benefit information was primarily influential, and there was no difference based on knowledge level.