  The recent investigations of pure Landau gauge SU(3) Yang-Mills theories
which are based on the truncated Schwinger-Dyson equations (SDE) indicate an
infrared power law behavior of the gluon and the ghost propagators. It has been
shown that the gluon propagator vanishes (or finite) in the infrared limit,
while the ghost propagator is more singular than a massless pole, and also that
there exists an infrared fixed point of the running gauge coupling. In this
paper we reexamine this picture by means of the exact (non-perturbative)
renormalization group (ERG) equations under some approximation scheme, in which
we treat not only two point functions but also four point vertices in the
effective average action with retaining their momentum dependence. Then it is
shown that the gluon and the ghost propagators with the infrared power law
behavior are obtained as an attractive solution starting from rather arbitrary
ultraviolet bare actions. Here it is found to be crucial to include the
momentum dependent four point vertices in the ERG framework, since otherwise
the RG flows diverge at finite scales. The features of the ERG analyses in
comparison with the SDE are also discussed.
