  We show that a relativistic gamma-ray burst (GRB) jet can potentially pierce
the envelope of very massive first generation star (Population III; Pop III) by
using the stellar density profile to estimate both the jet luminosity (via
accretion) and its penetrability. The jet breakout is possible even if the Pop
III star has a supergiant hydrogen envelope without mass loss, thanks to the
long-lived powerful accretion of the envelope itself. While the Pop III GRB is
estimated to be energetic, E_{gamma,iso} ~ 10^{55} erg, the supergiant envelope
hides the initial bright phase into the cocoon component, leading to a GRB with
a long duration ~ 1000(1+z) sec and an ordinary isotropic luminosity ~ 10^{52}
erg s^{-1} (~ 10^{-9} erg cm^{-2} s^{-1} at redshift z ~ 20). The
neutrino-annihilation is not effective for Pop III GRBs because of a low
central temperature, while the magnetic mechanism is viable. We also derive
analytic estimates of the breakout conditions, which are applicable to various
progenitor models. The GRB luminosity and duration are found to be very
sensitive to the core and envelope mass, providing possible probes of the first
luminous objects at the end of the high redshift dark ages.
