  Interplay of Pomeranchuk instability (spontaneous symmetry breaking of the
Fermi surface) and d-wave superconductivity is studied for the repulsive
Hubbard model on the square lattice with the dynamical mean field theory
combined with the fluctuation exchange approximation (FLEX+DMFT). We show that
the four-fold symmetric Fermi surface becomes unstable against a spontaneous
distortion into two-fold near the van Hove filling, where the symmetry of
superconductivity coexisting with the Pomeranchuk distorted Fermi surface is
modified from the d-wave pairing to (d+s)-wave. By systematically shifting the
position of van Hove filling with varied second- and third-neighbor hoppings,
we find that the transition temperature $T_{\rm c}^{\rm PI}$ of Pomeranchuk
instability is more sensitively affected by the position of van Hove filling
than the superconducting $T_{\rm c}^{\rm SC}$. This implies that the filling
region for strong Pomeranchuk instability and that for strong superconducting
fluctuations can be separated, and Pomeranchuk instability can appear even if
the peak of $T_c^{\rm PI}$ is lower than the peak of $T_c^{\rm SC}$. An
interesting observation is that the Fermi surface distortion can enhance the
superconducting $T_{\rm c}^{\rm SC}$ in the overdoped regime, which is
explained with a perturbation picture for small distortions.
