  We consider the effects of self-gravity on the hydrostatic balance in the
vertical direction of a gaseous disk and discuss the possible signature of the
self-gravity that may be captured by the direct imaging observations of
protoplanetary disks in future. In this paper, we consider a vertically
isothermal disk in order to isolate the effects of self-gravity. The specific
disk model we consider in this paper is the one with a radial surface density
gap, at which the Toomre's $Q$-parameter of the disk varies rapidly in the
radial direction. We calculate the vertical structure of the disk including the
effects of self-gravity. We then calculate the scattered light and the dust
thermal emission. We find that if the disk is massive enough and the effects of
self-gravity come into play, a weak bump-like structure at the gap edge appears
in the near-infrared (NIR) scattered light, while no such bump-like structure
is seen in the sub-mm dust continuum image. The appearance of the bump is
caused by the variation of the height of the surface in the NIR wavelength. If
such bump-like feature is detected in future direct imaging observations, with
the combination of sub-mm observations, it will bring us useful information
about the physical states of the disk.
