  Gamma-ray polarization offers a unique probes into the geometry of the
gamma-ray emission process in celestial objects. The Soft Gamma-ray Detector
(SGD) onboard the X-ray observatory Hitomi is a Si/CdTe Compton camera and is
expected to be an excellent polarimeter, as well as a highly sensitive
spectrometer due to its good angular coverage and resolution for Compton
scattering. A beam test of the final-prototype for the SGD Compton camera was
conducted to demonstrate its polarimetric capability and to verify and
calibrate the Monte Carlo simulation of the instrument. The modulation factor
of the SGD prototype camera, evaluated for the inner and outer parts of the
CdTe sensors as absorbers, was measured to be 0.649--0.701 (inner part) and
0.637--0.653 (outer part) at 122.2 keV and 0.610--0.651 (inner part) and
0.564--0.592 (outer part) at 194.5 keV at varying polarization angles with
respect to the detector. This indicates that the relative systematic
uncertainty of the modulation factor is as small as ~3%.
