  We have observed bursting variability of the 6.7 GHz methanol maser of
G33.641-0.228. Five bursts were detected in the observation period of 294 days
from 2009 to 2012. The typical burst is a large flux density rise in about one
day followed by a slow fall. A non-typical burst observed in 2010 showed a
large and rapid flux density enhancement from the stable state, but the rise
and fall of the flux density were temporally symmetric and a fast fluctuation
continued 12 days. On average, the bursts occurred once every 59 days, although
bursting was not periodic. Since the average power required for causing the
burst of order of 10^21 Js^-1 is far smaller than the luminosity of
G33.641-0.228, a very small fraction of the source's power would be sufficient
to cause the burst occasionally. The burst can be explained as a solar-flare
like event in which the energy is accumulated in the magnetic field of the
circumstellar disk, and is released for a short time. However, the mechanism of
the energy release and the dust heating process are still unknown.
