  Titan's stratosphere has been observed in a superrotation state, where the
atmosphere rotates many times faster than the surface does. Another
characteristics of Titan's atmosphere is the presence of thick haze layer. In
this paper, we performed numerical experiments using a General Circulation
Model (GCM), to explore the effects of the haze layer on the stratospheric
superrotation. We employed a semi-gray radiation model of Titan's atmosphere
following McKay et al. (1999), which takes account of the sunlight absorption
by haze particles. The phase change of methane or the seasonal changes were not
taken into account. Our model with the radiation parameters tuned for Titan
yielded the global eastward wind around the equator with larger velocities at
higher altitudes except at around 70 km after $10^5$ Earth days. Although the
atmosphere is not in an equilibrium state, the zonal wind profiles is
approximately consistent with the observed one. Analysis on our experiments
suggests that the quasi-stationary stratospheric superrotation is maintained by
the balance between the meridional circulation decoupled from the surface, and
the eddies that transport angular momentum equatorward. This is different from,
but similar to, the so-called Gierasch mechanism, in which momentum is supplied
from the surface. This structure may explain the no-wind region at about $80$
km in altitude.
