  We prove factorization and gauge invariance of the twist-3
single-spin-dependent cross section in the leading order perturbative QCD,
which was missing in the previous literature. We emphasize that the consistency
relation from the Ward identities for color gauge invariance is crucial to
guarantee the cancelation among the various gauge-noninvariant contributions in
the cross section. This relation also proves the absence of the "derivative"
terms in the cross section corresponding to the hard-pole and soft-fermion-pole
contributions. Applying the formalism to SIDIS, $ep^\uparrow\to e\pi X$, we
have derived the complete cross section associated with the twist-3
distribution for the transversely polarized nucleon.
