  We investigate the spin-triplet superconducting state of Sr2RuO4 in the
magnetic field along the c-axis on the basis of the four-component
Ginzburg-Landau (GL) model with a weak spin-orbit coupling. We consider
superconducting states described by the d-vector parallel to the ab-plane, and
find that three spin-triplet pairing states are stabilized in the magnetic
field-temperature phase diagram. Although a helical state is stable at low
magnetic fields, a chiral II state is stabilized at high magnetic fields. A
non-unitary spin-triplet pairing state appears near the transition temperature
owing to the coupling of magnetic field and chirality. We elucidate synergistic
and/or competing roles of the magnetic field, chirality, and spin-orbit
coupling. It is shown that a fractional vortex lattice is stabilized in the
chiral II phase owing to the spin-orbit coupling.
