The most definitive evidence for an accretion disk is provided by the discovery of megamasers (Claussen et al. 1984) which appear to be located in a molecular torus around the nucleus of the mildly active galaxy NGC 4258. Confirmation of Keplerian rotation speed was obtained from radio interferometer (Nakai et al. 1993) and VLBI observations (Greenhill, this volume). Based on the correlation between the spatial locations and radial velocities of the masers, Moran et al. (1995) deduced that the masers are located in a disk at radii R = 0.13–0.26 pc around a black hole with a mass M ≈ 3.5 × 107 M ⊙ (Watson & Wallin 1994, Maoz 1995). A limit on the thickness (H < 2.5 × 10−3 R) is deduced from the velocity dispersion of the maser sources. The mass-diffusion time scale is estimated to be τd > 1015 α−1 s, where α is the viscosity parameter. An efficient angular-momentum transfer mechanism (α > 0.1) is needed for τd ≈ 108 yr, which is the disk evolution time scale inferred from the correlation between interacting galaxies and intense AGN activities (MacKenty 1989, Hernquist 1989). A relatively large value of a is also required to account for the accretion rate needed to power the X-ray flux of NGC 4258.