We present observations of SDF-05M05, an unusual optical transient discovered in the Subaru Deep Field (SDF). The duration of the transient is > 800 days in the observer frame, and the maximum brightness during observation reached approximately 23mag in the i′ and z′ bands. The faint host galaxy is clearly identified in all five 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 -19mag for approximately 300days in the rest frame, which is significantly longer than a typical supernova and ultraluminous supernova. The total radiated energy during our observation was 1 × 1051 erg. The light curves and color evolution are marginally consistent with some luminous IIn supernovae. We suggest that the transient may be a unique and peculiar supernova at intermediate redshift. © 2012. The American Astronomical Society. All rights reserved..