  The Soft Gamma-ray Detector (SGD), to be deployed onboard the {\it ASTRO-H}
satellite, has been developed to provide the highest sensitivity observations
of celestial sources in the energy band of 60-600~keV by employing a detector
concept which uses a Compton camera whose field-of-view is restricted by a BGO
shield to a few degree (narrow-FOV Compton camera). In this concept, the
background from outside the FOV can be heavily suppressed by constraining the
incident direction of the gamma ray reconstructed by the Compton camera to be
consistent with the narrow FOV. We, for the first time, demonstrate the
validity of the concept using background data taken during the thermal vacuum
test and the low-temperature environment test of the flight model of SGD on
ground. We show that the measured background level is suppressed to less than
10\% by combining the event rejection using the anti-coincidence trigger of the
active BGO shield and by using Compton event reconstruction techniques. More
than 75\% of the signals from the field-of-view are retained against the
background rejection, which clearly demonstrates the improvement of
signal-to-noise ratio. The estimated effective area of 22.8~cm$^2$ meets the
mission requirement even though not all of the operational parameters of the
instrument have been fully optimized yet.
