  The disk around AB Aur was imaged and resolved at 24.6\,$\mu$m using the
Cooled Mid-Infrared Camera and Spectrometer on the 8.2m Subaru Telescope. The
gaussian full-width at half-maximum of the source size is estimated to be 90
$\pm$ 6 AU, indicating that the disk extends further out at 24.6\,$\mu$m than
at shorter wavelengths. In order to interpret the extended 24.6\,$\mu$m image,
we consider a disk with a reduced surface density within a boundary radius
$R_c$, which is motivated by radio observations that suggest a reduced inner
region within about 100 AU from the star. Introducing the surface density
reduction factor $f_c$ for the inner disk, we determine that the best match
with the observed radial intensity profile at 24.6\,$\mu$m is achieved with
$R_c$=88 AU and $f_c$=0.01. We suggest that the extended emission at
24.6\,$\mu$m is due to the enhanced emission from a wall-like structure at the
boundary radius (the inner edge of the outer disk), which is caused by a jump
in the surface density at $R_c$. Such reduced inner disk and geometrically
thick outer disk structure can also explain the more point-like nature at
shorter wavelengths. We also note that this disk geometry is qualitatively
similar to a pre-transitional disk, suggesting that the AB Aur disk is in a
pre-transitional disk phase.
