  Geometrical and physical properties of dusty tori of Seyfert nuclei probed by
the water vapor maser emission at 22 GHz are discussed. We assume that the
dusty torus has a simple cylindrical form and the maser emission can be
detected only when we observe the torus from almost edge-on views. The observed
low frequency of occurrence of the water vapor maser emission (less than 10
percent) suggests that the torus is a vertically thin cylinder whose outer
radius between a few pc and ~ 10 pc. However, the observed masing regions are
concentrated in the inner 1 pc regions of the torus. This property can be
explained by that only the inner a few pc regions have physical conditions
enough to cause the maser emission; the temperature is as high as several
hundred K and the density is as high as ~ 10^(10) cm^(-3).
