  Aims. We report on a detailed study of the optical afterglow of GRB 061121
with our original time-series photometric data. In conjunction with X-ray
observations, we discuss the origin of its optical and X-ray afterglows.
Methods. We observed the optical afterglow of Swift burst GRB 061121 with the
Kanata 1.5-m telescope at Higashi-Hiroshima Observatory. Our observation covers
a period just after an X-ray plateau phase. We also performed deep imaging with
the Subaru telescope in 2010 in order to estimate the contamination of the host
galaxy. Results. In the light curve, we find that the optical afterglow also
exhibited a break as in the X-ray afterglow. However, our observation suggests
a possible hump structure or a flattening period before the optical break in
the light curve. There is no sign of such a hump in the X-ray light curve.
Conclusions. This implies that the emitting region of optical was distinct from
that of X-rays. The hump in the optical light curve was possibly caused by the
passage of the typical frequency of synchrotron emission from another forward
shock distinct from the early afterglow. The observed decay and spectral
indices are inconsistent with the standard synchrotron-shock model. Hence, the
observation requires a change in microphysical parameters in the shock region
or a prior activity of the central engine. Alternatively, the emission during
the shallow decay phase may be a composition of two forward shock emissions, as
indicated by the hump structure in the light curve.
