  We analyze the single transverse-spin asymmetry (SSAs) for inclusive hadron
production in the $pp$ collision at RHIC based on the twist-3 mechanism in QCD.
As an origin of SSAs, we take into account of all kinds of pole contributions
associated with the twist-3 quark-gluon correlation functions in the polarized
proton. By the inclusion of the soft-fermion-pole contribution in addition to
the soft-gluon-pole contribution, we find SSAs observed at RHIC are better
described for all kinds of mesons. $P_T$-dependence of the asymmetry and the
comparison with the FNAL data are also presented.
