According to existing marketing research and conventional wisdom, advertising is known to provide benefits to the advertised brand (hereinafter the target brand) while causing harm to competitor brands. However, if the increase in attitudes and preferences caused by advertising occurs not only for the target brand but also for competitor brands, what logic can explain this phenomenon? Based on the associative network model, this study sought to elucidate how the "competitor-benefiting effect of advertising" can occur and under what market structure conditions this phenomenon becomes more pronounced. According to associative network theory, brands within the same category exist as nodes in consumers' knowledge structures, and these nodes are interconnected to form a network. Furthermore, according to the spreading activation model, because the associative relationship between brands creates the possibility that the activation of one brand may lead to the activation of another, when consumers are exposed to advertising for one brand within the same category, the nodes corresponding to other brands may also be activated, thereby producing the competitor-benefiting effect of advertising. Since the competitor-benefiting effect of advertising originates from the associative relationships between brands as nodes in the associative network structure, the magnitude of the effect would depend on the strength of these associative relationships. According to the spreading activation model, one factor influencing the strength of these associative relationships is the number of nodes; therefore, the competitor-benefiting effect of advertising is expected to be influenced by the number of competing brands. This study hypothesized that as the number of competing brands decreases, both the likelihood and the magnitude of the competitor-benefiting effect of advertising would increase. To empirically test this, purchase intentions for the target brand, competitor brands, and the relevant category were measured and compared before and after advertising for the target brand, in categories with many versus few competitor brands. To control for the influence of the target brand's status on the experimental results, the target brand was consistently set as a minor brand throughout the experiment. The results confirmed the competitor-benefiting effect of advertising—that is, advertising increased consumers' purchase intentions not only for the target brand but also for competitor brands within the same category. Moreover, in categories with fewer competitors, both the likelihood and magnitude of the competitor-benefiting effect were found to be relatively higher. In other words, the existence and magnitude of the competitor-benefiting effect of advertising were found to be moderated by the number of competing brands. This study explored the possibility and conditions under which advertising can positively influence not only the target brand but also competitor brands in terms of attitudes in competitive situations, from the perspectives of the associative network model and market structure. Its significance lies in being the first attempt to theoretically articulate and experimentally demonstrate the competitor-benefiting effect of advertising.