  Just as a residue field can be considered for a point of an algebraic
variety, we can also consider a residue field for a point of a Berkovich
analytic space. This residue field is a valuation field in the algebraic sense.
Then we can consider its residue field as a valuation field. We call it the
Berkovich double residue field at the point. In this paper, we consider a point
$x$ of the Berkovich analytification of an algebraic variety and identify the
Berkovich double residue field at $x$ with the union of the residue fields at
the center of $x$ in birational models. Besides, we concretely compute the
Berkovich double residue field for any quasi monomial valuation.
