  We have performed near-infrared monitoring observations of Sgr A*, the
Galactic center radio source associated with a supermassive black hole, with
the near-infrared camera CIAO and the 36-element adaptive optics system on the
Subaru telescope. We observed three flares in the Ks band (2.15micron) during
220 min monitoring on 2008 May 28, and confirmed the flare emission is highly
polarized, supporting the synchrotron radiation nature of the near-infrared
emission. Clear variations in the degree and position angle of polarization
were also detected: an increase of the degree of polarization of about 20 %,
and a swing of the position angle of about 60 - 70 degrees in the declining
phase of the flares. The correlation between the flux and the degree of
polarization can be well explained by the flare emission coming from hotspot(s)
orbiting Sgr A*. Comparison with calculations in the literature gives a
constraint to the inclination angle i of the orbit of the hotspot around Sgr
A*, as 45 < i < 90 degrees (close to edge-on).
