  The three-state Potts model with antiferromagnetic nearest-neighbor (n.n.)
and ferromagnetic next-nearest-neighbor (n.n.n) interaction is investigated
within a mean-field theory. We find that the phase-diagram contains two kind of
ordered phases, so-called BSS phase and PSS phase, separated by a discontinuous
phase transition line. Order-disorder transition is continuous for the weak
n.n.n. interaction and becomes discontinuous transition when the n.n.n.
interaction is increased. We show that the multicritical point where the
order-disorder transition becomes discontinuous is indeed a tricritical point.
