  We obtain an almost perfect monopole action numerically after abelian
projection in pure SU(3) lattice QCD. Performing block-spin transformations on
the dual lattice, the action fixed depends only on a physical scale b. Monopole
condensation occurs for large b region. The numerical results show that
two-point monopole interactions are dominant for large b. We next perform the
block-spin transformation analytically in a simplified case of two-point
monopole interactions with a Wilson loop on the fine lattice. The perfect
operator evaluating the static quark potential on the coarse b-lattice are
derived. The monopole partition function can be transformed into that of the
string model. The static potential and the string tension are estimated in the
string model framework. The rotational invariance of the static potential is
recovered, but the string tension is a little larger than the physical one.
