Supermassive black holes observed at high redshift $zgtrsim6$ could grow from direct collapse black holes (DCBHs) with mass $sim10^5,M_{odot}$, which result from the collapse of supermassive stars (SMSs). If a relativistic jet is launched from a DCBH, it can break out of the collapsing SMS and produce a gamma-ray burst (GRB). Although most of the GRB jets are off-axis from our line of sight, we show that the energy injected from the jet into a cocoon is huge $sim10^{55-56},{rm{erg}}$, so that the cocoon fireball is observed as ultra-luminous supernovae of $sim10^{45-46}rm{,erg,s^{-1}}$ for $sim5000 [(1+z)/16] rm{,days}$. They are detectable by the future telescopes with near infrared bands, such as, $Euclid$, $WFIRST$, $WISH$, and $JWST$ up to $zsim20$ and $sim 100$ events per year, providing a direct evidence of the DCBH scenario.
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