This study examined how attitudes toward extension products and beliefs about the parent brand following brand extension vary depending on the perceived breadth of the parent brand and the product category similarity of the extension product. Furthermore, these topics were investigated under two conditions: when subjects were merely informed that a brand extension had occurred (Experiment 1), and when subjects were additionally provided with information about the initial market performance (success or failure) of the extension product (Experiment 2). The results of Experiment 1 showed that when subjects perceived the parent brand breadth as wide, attitudes toward the extension product were similarly favorable regardless of the degree of extension product similarity, whereas when subjects perceived the parent brand breadth as narrow, favorability toward the extension product declined when similarity was very low. Meanwhile, post-extension belief evaluations of the parent brand were not affected regardless of the extension product's similarity level when subjects perceived the parent brand breadth as wide, whereas when subjects perceived the parent brand breadth as narrow, beliefs were negatively affected when the product extension was made into a somewhat dissimilar domain. The results of Experiment 2 showed that when information indicated the extension product's initial market performance was successful, parent brand beliefs remained unchanged regardless of the extension product's similarity level. In contrast, when information indicated the extension product's initial performance was pessimistic, a dilution effect—whereby parent brand beliefs were negatively affected—occurred only in the condition where the parent brand breadth was perceived as narrow and the extension product's similarity was somewhat low. Conversely, for subjects who perceived the parent brand breadth as wide, no such dilution effect appeared regardless of extension product similarity. Finally, the theoretical and practical contributions of these findings are discussed.