  We propose a new diagnostic of the ``dusty torus'' model for Seyfert nuclei.
Dust grains in the torus are heated by the nuclear continuum, and reradiate
mostly in the mid-infrared wavelengths. From the torus geometry, it is
predicted that the emission at lambda < 10 micron has strong dependence on the
viewing angle. Since the dependence is predicted to be insignificant at lambda
> 10 micron, we study the flux ratio between 3.5 micron (L band) and 25 micron;
R(L,25) = log [(nu_3.5 um S_nu_3.5 um)/(nu_25 um S_nu_25 um)]. In three
different samples (optically selected, X-ray selected, and infrared selected
samples) of Seyfert galaxies, the observed values of R(L,25) between type 1
Seyferts (S1s) and type 2 Seyferts (S2s) are found to be clearly separated;
R(L,25) > -0.6 for S1s while R(L,25) < -0.6 for S2s. This implies universality
of their torus properties. With this result and the other observational
characteristics, we investigate the most plausible torus model among those
presented in Pier & Krolik (1992, 1993).
