  We have performed monitoring observations of the flux density toward the
Galactic center compact radio source, Sagittarius A* (Sgr A*), which is a
supermassive black hole, from 1996 to 2005 using the Nobeyama Millimeter Array
of the Nobeyama Radio Observatory, Japan. These monitoring observations of Sgr
A* were carried out in the 3- and 2-mm (100 and 140 GHz) bands, and we have
detected several flares of Sgr A*. We found intraday variation of Sgr A* in the
2000 March flare. The twofold increase timescale is estimated to be about 1.5
hr at 140 GHz. This intraday variability suggests that the physical size of the
flare-emitting region is compact on a scale at or below about 12 AU (~150 Rs;
Schwarzschild radius). On the other hand, clear evidence of long-term periodic
variability was not found from a periodicity analysis of our current millimeter
data set.
