  We discuss the dust distribution within photoionized regions. Assuming a
geometry with a central dust cavity, which is strongly suggested by the
literature, we can estimate the cavity's radius from the ratio of the infrared
and radio fluxes by using a simple transfer model of Lyman continuum photons.
We apply the method to a sample of the Galactic H {\sc ii} regions. The
estimated typical radius of the dust cavity of the Galactic compact H {\sc ii}
regions is about 30% of the Str{\"o}mgren radius. Taking account of
uncertainties both of the observational data and the model, we can reject a
dust distribution lacking a central cavity. Therefore, the dust cavity model is
supported independently of the previous works. We discuss the formation
mechanism of such a dust cavity and its detectability by present and future
infrared facilities.
