  We present observations of SDF-05M05, an unusual optical transient discovered
in the Subaru Deep Field (SDF). The duration of the transient is > ~800 d in
the observer frame, and the maximum brightness during observation reached
approximately 23 mag in the i' and z' bands. The faint host galaxy is clearly
identified in all 5 optical bands of the deep SDF images. The photometric
redshift of the host yields z~0.6 and the corresponding absolute magnitude at
maximum is ~-20. This implies that this event shone with an absolute magnitude
brighter than -19 mag for approximately 300 d in the rest frame, which is
significantly longer than a typical supernova and ultra-luminous supernova. The
total radiated energy during our observation was 1x10^51 erg. The light curves
and color evolution are marginally consistent with some of luminous IIn
supernova. We suggest that the transient may be a unique and peculiar supernova
at intermediate redshift.
