  In this paper, we assume that all isoparametric submanifolds have flat
section. The main purpose of this paper is to prove that, if a full irreducible
complete isoparametric submanifold of codimension greater than one in a
symmetric space of non-compact type admits a reflective focal submanifold and
if it of real analytic, then it is a principal orbit of a Hermann type action
on the symmetric space. A hyperpolar action on a symmetric space of non-compact
type admits a reflective singular orbit if and only if it is a Hermann type
action. Hence is not extra the assumption that the isoparametric submanifold
admits a reflective focal submanifold. Also, we prove that, if a full
irreducible complete isoparametric submanifold of codimension greater than one
in a symmetric space of non-compact type satisfies some additional conditions,
then it is a principal orbit of the isotropy action of the symmetric space,
where we need not impose that the submanifold is of real analytic. We use the
building theory in the proof.
