  We investigated the size distribution of high-inclination main-belt asteroids
(MBAs) to explore asteroid collisional evolution under hypervelocity collisions
of around 10 km/s. We performed a wide-field survey for high-inclination sub-km
MBAs using the 8.2-m Subaru Telescope with the Subaru Prime Focus Camera
(Suprime-Cam). Suprime-Cam archival data were also used. A total of 616 MBA
candidates were detected in an area of 9.0 deg^2 with a limiting magnitude of
24.0 mag in the SDSS r filter. Most of candidate diameters were estimated to be
smaller than 1 km. We found a scarcity of sub-km MBAs with high inclination.
Cumulative size distributions (CSDs) were constructed using Subaru data and
published asteroid catalogs. The power-law indexes of the CSDs were 2.17 +/-
0.02 for low-inclination (< 15 deg) MBAs and 2.02 +/- 0.03 for high-inclination
(> 15 deg) MBAs in the 0.7-50 km diameter range. The high-inclination MBAs had
a shallower CSD. We also found that the CSD of S-like MBAs had a small slope
with high inclination, whereas the slope did not vary with inclination in the
C-like group. The most probable cause of the shallow CSD of the
high-inclination S-like MBAs is the large power-law index in the
diameter-impact strength curve in hypervelocity collisions. The collisional
evolution of MBAs may have advanced with oligopolistic survival during the
dynamical excitation phase in the final stage of planet formation.
