  We study the contribution of the twist-3 fragmentation function to the
single-transverse spin asymmetry in semi-inclusive deep inelastic scattering
within the framework of the collinear factorization. Making use of the
Ward-Takahashi identity in QCD, we establish the formalism in the Feynman gauge
to calculate the non-pole contribution of the twist-3 fragmentation function to
the asymmetry and derive the complete cross-section formula in the leading
order QCD perturbation theory. The obtained twist-3 hadronic tensor is shown to
satisfy the electromagnetic gauge-invariance. The behavior in the small
transverse- momentum region for the five structure functions is also given.
