  To solve the hierarchy problem of the Higgs mass, it may be suggested that
there are no an intermediate scale up to the Planck scale except for the TeV
scale. For this motivation, we investigate possibilities of gauge coupling
unification (GCU) at the Planck scale ($M_{Pl} = 2.4 \times 10^{18}\,{\rm
GeV}$) by adding extra particles with the TeV scale mass into the standard
model. We find that the GCU at the Planck scale can be realized by extra
particles including some relevant scalars, while it cannot be realized only by
extra fermions with the same masses. On the other hand, when extra fermions
have different masses, the GCU can be realized around $\sqrt{8 \pi} M_{Pl}$. By
this extension, the vacuum can become stable up to the Planck scale.
