Innovation in high-technology products is influenced not only by innovation in the products themselves but also by other products or services with complementary functions, which is referred to as complementary innovation and primarily manifests in the relationships between high-technology hardware and software or between hardware and content. As an example of such complementary innovation, in the eBook industry, because content providers and device providers are in a mutually complementary relationship, it is not a straightforward matter to prescribe investment and market penetration strategies for each party. Furthermore, the impact of digital rights management (DRM) to prevent illegal copying of digital content on diffusion has not been clearly established. Accordingly, this study proposed an agent-based model for the eBook industry and conducted various simulation experiments using this model. The following research propositions were derived from the results. First, if a software provider's investment does not exceed a certain level in the complementary innovation process, innovation diffusion fails in its early stages. In other words, a critical threshold exists for the software provider's investment to achieve successful complementary innovation. Second, the investment utility of the hardware provider exhibits diminishing returns in the complementary innovation process between software and hardware. Therefore, an optimal investment level exists at which the hardware provider can maximize profits. Third, the impact of DRM on digital content market diffusion varies depending on the content provider's strategy. Through these findings, the study presented market penetration strategies for stakeholders participating in complementary innovation and contributed to clarifying the relationships among them.