  We verified the off-axis jet model of X-ray flashes (XRFs) and examined a
discovery of off-axis orphan gamma-ray burst (GRBs) afterglows. The XRF sample
was selected on the basis of the following three factors: (1) a constraint on
the lower peak energy of the prompt spectrum $E^{src}_{obs}$, (2) redshift
measurements, and (3) multi-color observations of an earlier (or brightening)
phase. XRF020903 was the only sample selected basis of these criteria. A
complete optical multi-color afterglow light curve of XRF020903 obtained from
archived data and photometric results in literature showed an achromatic
brightening around 0.7 days. An off-axis jet model with a large observing angle
(0.21 rad, which is twice the jet opening half-angle, $\theta_{jet}$) can
naturally describe the achromatic brightening and the prompt X-ray spectral
properties. This result indicates the existence of off-axis orphan GRB
afterglow light curves. Events with a larger viewing angle
($>\sim2\theta_{jet}$) could be discovered using an 8-m class telescope with
wide field imagers such as Subaru Hyper-Suprime-Cam and the Large Synoptic
Survey Telescope.
