  We investigate the nature of the phase transition of the ferromagnetic Potts
model with invisible states. The ferromagnetic Potts model with invisible
states can be regarded as straightforward extension of the standard
ferromagnetic Potts model. The invisible states contribute the entropy, however
they do not affect the internal energy. They also do not change the symmetry
which breaks at the transition temperature. The invisible states stimulate a
first-order phase transition. We confirm that the first-order phase transition
with spontaneous $q$-fold symmetry breaking for $q=2,3$, and 4 takes place even
on two-dimensional lattice by Monte Carlo simulation. We also find that the
transition temperature decreases and the latent heat increases as the number of
invisible states increases.
