This chapter examines the regulatory landscape of digital markets, focusing on the shift from ex post to ex ante enforcement. Using a stock price event study, it investigates how markets assess the costs of ex ante regulation, particularly under Japan’s Digital Platform Transparency Act. The analysis shows that each major regulatory step—such as cabinet approval, enactment, and designation of covered firms—led to stock market declines of 10–20% for affected companies. These findings suggest that ex ante regulation is perceived as imposing broad and significant compliance costs, in contrast to ex post enforcement, which targets individual firms and produces narrower effects. The results highlight that while ex ante regulation aims to preempt anticompetitive behavior, it also generates substantial business burdens that exceed those associated with ex post approaches. Recognizing these trade-offs underscores the need for careful oversight and rigorous evaluation to ensure that proactive regulation effectively safeguards competition without imposing disproportionate costs on market participants.