  A discrete R-symmetry often appears as an exact gauge symmetry in the low
energy effective theory of superstring theories. We search for such discrete
R-symmetries from a phenomenological point of view and find that Z_{9R} and
Z_{18R} are candidates of the nonanomalous R-symmetry in the case of the
minimal supersymmetric standard model. We also find Z_{4R} and Z_{20R} in the
case that quarks and leptons are embedded in the SU(5) GUT multiplets.
Interesting is that in the latter case all the solutions predict some extra
matter multiplets and we find that the simplest choice is to take a pair of
{\bf 5} and {\bf 5}^* of SU(5)_{GUT} whose mass is of order the SUSY breaking
scale \sim 1 TeV. We emphasize that the presence of such extra matters is
testable in future collider experiments.
