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We present optical spectroscopic and photometric observations of supernova(SN) 2008D, associated with the luminous X-ray transient 080109, at >300 daysafter the explosion (nebular phases). We also give flux measurements ofemission lines from the H II region at the site of the SN, and estimates of thelocal metallicity. The brightness of the SN at nebular phases is consistentwith the prediction of the explosion models with an ejected 56Ni mass of 0.07Msun, which explains the light curve at early phases. The [O I] line in thenebular 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 anaspherical explosion. The profile can be explained by a torus-like distributionof oxygen viewed from near the plane of the torus. We suggest that SN 2008D isa side-viewed, bipolar explosion with a viewing angle of > 50^{\circ} from thepolar direction.

Nebular Phase Observations of the Type Ib Supernova 2008D/X-ray Transient 080109: Side-Viewed Bipolar Explosion