  The Hard X-ray Detector (HXD) on board Suzaku covers a wide energy range from
10 keV to 600 keV by combination of silicon PIN diodes and GSO scintillators.
The HXD is designed to achieve an extremely low in-orbit back ground based on a
combination of new techniques, including the concept of well-type active shield
counter. With an effective area of 142 cm^2 at 20 keV and 273 cm2 at 150 keV,
the background level at the sea level reached ~1x10^{-5} cts s^{-1} cm^{-2}
keV^{-1} at 30 keV for the PI N diodes, and ~2x10^{-5} cts s^{-1} cm^{-2}
keV^{-1} at 100 keV, and ~7x10^{-6} cts s^{-1} cm^{-2} keV^{-1} at 200 keV for
the phoswich counter. Tight active shielding of the HXD results in a large
array of guard counters surrounding the main detector parts. These
anti-coincidence counters, made of ~4 cm thick BGO crystals, have a large
effective area for sub-MeV to MeV gamma-rays. They work as an excellent
gamma-ray burst monitor with limited angular resolution (~5 degree). The
on-board signal-processing system and the data transmitted to the ground are
also described.
