  We report the detection of bursts of 6.7 GHz methanol maser emission in a
high-mass star-forming region, G33.64-0.21. One of the spectral components of
the maser in this source changed its flux density by 7 times that of the
previous day, and it decayed with a timescale of 5 days. The burst occurred
repeatedly in the spectral component, and no other components showed such
variability. A VLBI observation with the Japanese VLBI Network (JVN) showed
that the burst location was at the southwest edge of a spatial distribution,
and the bursting phenomenon occurred in a region much smaller than 70 AU. We
suggest an impulsive energy release like a stellar flare as a possible
mechanism for the burst. These results imply that 6.7 GHz methanol masers could
be a useful new probe for studying bursting activity in the process of star
formation of high-mass YSOs with a high-resolution of AU scale.
