  From the point of view of the gauge hierarchy problem, introducing an
intermediate scale in addition to TeV scale and the Planck scale ($M_{\rm Pl} =
2.4 \times 10^{18}\,{\rm GeV}$) is unfavorable. In that way, a gauge coupling
unification (GCU) is expected to be realized at $M_{\rm Pl}$. We explore
possibilities of GCU at $M_{\rm Pl}$ by adding a few extra particles with TeV
scale mass into the standard model (SM). When extra particles are fermions and
scalars (only fermions) with the same mass, the GCU at $M_{\rm Pl}$ can (not)
be realized. On the other hand, when extra fermions have different masses, the
GCU can be realized around $\sqrt{8 \pi} M_{\rm Pl}$ without extra scalars.
This simple SM extension has two advantages that a vacuum becomes stable up to
$M_{\rm Pl}$ ($\sqrt{8 \pi} M_{\rm Pl}$) and a proton lifetime becomes much
longer than an experimental bound.
