  Establishing the twist-3 formalsim for the single transverse spin asymmetry,
we present a complete single-spin-dependent cross section for SIDIS,
$ep^\uparrow\to e\pi X$, associated with the twist-3 distribution for the
transversely polarized nucleon. We emphasize that the consistency condition
from the Ward identities for color gauge invariance is crucial to prove
factorization property of the cross section.
