  We performed a wide-field survey observation of small asteroids using the
Hyper Suprime-Cam installed on the 8.2 m Subaru Telescope. We detected more
than 3,000 main-belt asteroids with a detection limit of 24.2 mag in the
r-band, which were classified into two groups (bluish C-like and reddish
S-like) by the g-r color of each asteroid and obtained size distributions of
each group. We found that the shapes of size distributions of asteroids with
the C-like and S-like colors agree with each other in the size range of 0.4-5
km in diameter. Assuming the asteroid population in this size range is under
collision equilibrium, our results indicate that compositional difference
hardly affects the size dependence of impact strength, at least for the size
range between several hundred meters and several kilometers. This size range
corresponds to the size range of ``spin-barrier'', an upper limit observed in
the rotation rate distribution. Our results are consistent with the view that
most asteroids in this size range have a rubble-pile structure.
