  We present optical spectroscopic and photometric observations of supernova
(SN) 2008D, associated with the luminous X-ray transient 080109, at >300 days
after the explosion (nebular phases). We also give flux measurements of
emission lines from the H II region at the site of the SN, and estimates of the
local metallicity. The brightness of the SN at nebular phases is consistent
with the prediction of the explosion models with an ejected 56Ni mass of 0.07
Msun, which explains the light curve at early phases. The [O I] line in the
nebular spectrum shows a double-peaked profile while the [Ca II] line does not.
The double-peaked [O I] profile strongly indicates that SN 2008D is an
aspherical explosion. The profile can be explained by a torus-like distribution
of oxygen viewed from near the plane of the torus. We suggest that SN 2008D is
a side-viewed, bipolar explosion with a viewing angle of > 50^{\circ} from the
polar direction.
