As exemplified above, the areas of diffusion research are being broadened, even to the relationship between diffusion and economic growth (Benhabib and Spiegel, 2005 and Czernich et al., 2009). The effect of network neutrality regulation, which this paper investigates, will also be changed according to the level of diffusion with respect to the current number of adopters. However, no research has been conducted regarding the diffusion of new Internet application services, and previous research even assumed that the market potential and final number of adopters would be constant at the current level of market penetration.