  We report optical observations during the first hour of the gamma-ray burst
(GRB) afterglow of GRB021004. Our observation revealed the existence of a short
plateau phase, in which the afterglow remained at almost constant brightness,
before an ordinary rapid fading phase. This plateau phase lasted for about 2
hours from 0.024 to 0.10 d after the burst, which corresponds to a missing
blank of the early afterglow light curve of GRB990123. We propose that the
plateau phase can be interpreted as the natural evolution of synchrotron
emission from the forward shock region of a blast wave. The time when the
typical frequency of the synchrotron emission passes through the optical range
has been predicted to be about 0.1 d after the burst, which is consistent with
the observed light curve. Our scenario hence implies that the observed feature
in GRB021004 is a common nature of GRB afterglows.
