  The nuclear saturation mechanism is discussed in terms of two-nucleon and
three-nucleon interactions in chiral effective field theory (Ch-EFT), using the
framework of lowest-order Brueckner theory. After the Coester band, which is
observed in calculating saturation points with various nucleon-nucleon (NN)
forces, is revisited using modern NN potentials and their low-momentum
equivalent interactions, detailed account of the saturation curve of the Ch-EFT
interaction is presented. The three-nucleon force (3NF) is treated by reducing
it to an effective two-body interaction by folding the third nucleon degrees of
freedom. Uncertainties due to the choice of the 3NF low-energy constants $c_D$
and $c_E$ are discussed. The reduction of the cutoff-energy dependence of the
NN potential is explained by demonstrating the effect of the 3NF in the
$^1$S$_0$ and $^3$S$_1$ states.
