  In this paper we show that the equation of motion of a massless scalar
particle near the black hole (BH) horizon in the Schwarzschild-deSitter (SdS)
spacetime enjoys a hidden conformal symmetry when the two horizons satisfy a
quadratic relation, not related with the Nariai limit. This hidden symmetry is
$SL(2,\mathbb{R})\times SO(2)$ instead of $SO(2,1)\times SO(2)$, the expected
symmetry of $dS_2\times S^2$. We present a structural analysis of SdS spacetime
and compute the leading quantum corrections to the entropy of SdS spacetime in
a semi-classical way.
