  We carried out low-resolution optical spectroscopy of 51 main-belt asteroids,
most of which have highly-inclined orbits. They are selected from D-type
candidates in the SDSS-MOC 4 catalog. Using the University of Hawaii 2.2 m
telescope and the Inter-University Centre for Astronomy and Astrophysics 2 m
telescope in India, we determined the spectral types of 38 asteroids. Among
them, eight asteroids were classified as D-type asteroids. Fractions of D-type
asteroids are 3.0+/-1.1 for low orbital inclination main-belt asteroids and
7.3+/-2.0 for high orbital inclination main-belt asteroids. The results of our
study indicate that some D-type asteroids were formed within the ecliptic
region between the main belt and Jupiter, and were then perturbed by Jupiter.
