  We discuss the connection between black hole and holographic dark energy. We
examine the issue of the equation of state (EOS) for holographic energy density
as a candidate for the dark energy carefully. This is closely related to the
EOS for black hole, because the holographic dark energy comes from the black
hole energy density. In order to derive the EOS of a black hole, we may use its
dual (quantum) systems. Finally, a regular black hole without the singularity
is introduced to describe an accelerating universe inside the cosmological
horizon. Inspired by this, we show that the holographic energy density with the
cosmological horizon as the IR cutoff leads to the dark energy-dominated
universe with $\omega_{\rm \Lambda}=-1$.
