We report the detection of an extremely bright ($sim$37 mJy at 1100 $mu$m and $sim$91 mJy at 880 $mu$m) submillimeter galaxy (SMG), AzTEC-ASTE-SXDF1100.001 (hereafter referred to as SXDF1100.001 or Orochi), discovered in 1100 $mu$m observations of the Subaru/XMM-Newton Deep Field using AzTEC on ASTE. Subsequent CARMA 1300 $mu$m and SMA 880 $mu$m observations successfully pinpoint the location of Orochi and suggest that it has two components, one extended (FWHM of $sim$ 4$^{primeprime}$) and one compact (unresolved). Z-Spec on CSO has also been used to obtain a wide band spectrum from 190 to 308 GHz, although no significant emission/absorption lines are found. The derived upper limit to the line-to-continuum flux ratio is 0.1--0.3 (2 $sigma$) across the Z-Spec band. Based on the analysis of the derived spectral energy distribution from optical to radio wavelengths of possible counterparts near the SMA/CARMA peak position, we suggest that Orochi is a lensed, optically dark SMG lying at $z sim 3.4$ behind a foreground, optically visible (but red) galaxy at $z sim 1.4$. The deduced apparent (i.e., no correction for magnification) infrared luminosity ($L_{rm IR}$) and star formation rate (SFR) are $6 times 10^{13}$ $L_{odot}$ and 11000 $M_{odot}$ yr$^{-1}$, respectively, assuming that the $L_{rm IR}$ is dominated by star formation. These values suggest that Orochi will consume its gas reservoir within a short time scale ($3 times 10^{7}$ yr), which is indeed comparable to those in extreme starbursts like the centres of local ULIRGs.
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