  We present a SED model of dusty galaxies, in which the equation of radiative
transfer is solved by assuming spherical symmetry. The temperature fluctuation
of very small dust particles is calculated consistently with the radiative
transfer. The adopted dust model consists of graphite and silicate grains and
PAHs, whose relative fractions are determined for each MW, LMC and SMC type
extinction curve. This model allows us to derive the intrinsic SEDs of stellar
populations embedded in dusty ISM, which are very important indicators for the
age of stellar populations. Therefore, the evolutionary phase of starburst
galaxies which have frequently very dusty ISM can be investigated with this SED
model. We show that the SEDs of Arp220 and M82 can both be explained by the
same single stellar population, despite the significant differences in the SEDs
and the infrared luminosities. The apparent difference between their SEDs is
mainly caused by the difference in the optical depth. In contrast, the SED of
prototypical star-forming ERO, HR10, indicates that this galaxy is relatively
old comparing to Arp220 and M82. It is found that, in the case of optically
thin limit like elliptical galaxies, the optical depth cannot be inferred only
from the SED, due to a degeneracy between the optical depth, galactic size, and
the spatial distribution of dust; the latter two are important for estimating
the average temperature of dust grains in elliptical galaxies. When the
observed size of elliptical galaxies is adopted for the model geometry, SEDs
can be used to constrain the spatial distribution of dust in elliptical
galaxies.
