  Effects of the band structure on the symmetry of superconducting (SC) states
are studied. For a square lattice system with a nearest-neighbor attractive
interaction, SC states with various symmetries are found by changing the band
structure, or, the shape of the Fermi surface. The spin-triplet ($(p_x +
ip_y)$-wave) and spin-singlet ($d$- or s-wave) SC states, and states with their
coexistence ($d + ip_y$, $s + ip_y$) can be stabilized within the same type of
interaction. The stability of interlayer-pairing states with line nodes is also
examined, and its relation to the SC state of Sr$_2$RuO$_4$ is discussed.
