  The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state in quasi-one-dimensional
systems with warped Fermi surfaces is examined in strong parallel magnetic
fields. It is shown that the state is extremely stable for field directions
around nontrivial optimum directions, at which the upper critical field
exhibits cusps, and that the stabilization is due to a Fermi-surface effect
analogous to the nesting effect for the spin density wave and charge density
wave. Interestingly, the behavior with cusps is analogous to that in a square
lattice system in which the hole density is controlled. For the organic
superconductor (TMTSF)_2ClO_4, when the hopping parameters obtained by previous
authors based on X-ray crystallography results are assumed, the optimum
directions are in quadrants consistent with the previous experimental
observations. Furthermore, near this set of parameters, we also find sets of
hopping parameters that more precisely reproduce the observed optimum in-plane
field directions. These results are consistent with the hypothesis that the
FFLO state is realized in the organic superconductor.
