  Transfer equation in a geometrically thin accretion disk is reexamined under
the plane-parallel approximation with finite optical depth. Emergent intensity
is analytically obtained in the cases with or without internal heating. For
large or infinite optical depth, the emergent intensity exhibits a usual
limb-darkening effect, where the intensity linearly changes as a function of
the direction cosine. For small optical depth, on the other hand, the
angle-dependence of the emergent intensity drastically changes. In the case
without heating but with uniform incident radiation at the disk equator, the
emergent intensity becomes isotropic for small optical depth. In the case with
uniform internal heating, the limb brightening takes place for small optical
depth. We also emphasize and discuss the limb-darkening effect in an accretion
disk for several cases.
