The adoption of U.S.-style independent regulatory agencies (IRAs) such as the FCC and the SEC has become increasingly pervasive in advanced nations. Much recent political analysis of public regulation tends to regard this worldwide spread of the American model, or “regulatory state,” as a typical example of a policy diffusion mechanism, such as emulation or regulatory competition. After critically examining that view, this article develops an alternative framework that combines international structural forces, such as techno-economic changes and ideational factors, and domestic institutional factors, in particular, path dependence of historical institutions. The cross-national and cross-sectoral case analysis (the financial services and communications sector in Britain and Japan) demonstrates that while apparent convergence on the American model is due to international structural forces, national and sectoral differences with respect to the extent of organizational changes depend largely on domestic institutions. This finding suggests that, facing complex dynamism between international structural forces and domestic institutions, each national government chooses the feature of regulatory organization independently from other nation’s decisions, rather than interdependently as assumed in the policy diffusion approach.