  For a spherically symmetric vacuum model with a negative cosmological
constant, a complex constrained instanton is considered as the seed for the
quantum pair creation of Schwarzschild-anti-de Sitter black holes. The relative
creation probability is found to be the exponential of the negative of the
black hole entropy. The black hole entropy is known to be one quarter of the
black hole horizon area. In the absence of a general no-boundary proposal for
open creation, the constrained instanton approach is used in treating both the
open and closed pair creations of black holes.
