  Three-nucleon forces, which compose an up-to-date subject in few-nucleon
systems, provide a good account of the triton binding energy and the cross
section minimum in proton-deuteron elastic scattering, while do not succeed in
explaining spin observables such as the nucleon and deuteron analyzing powers,
suggesting serious defects in their spin dependence. We study the spin
structure of nucleon-deuteron elastic amplitudes by decomposing them into
spin-space tensors and examine effects of three-nucleon forces to each
component of the amplitudes obtained by solving the Faddeev equation. Assuming
that the spin-scalar amplitudes dominate the others, we derive simple
expressions for spin observables in the nucleon-deuteron elastic scattering.
The expressions suggest that a particular combination of spin observables in
the scattering provides direct information of scalar, vector, or tensor
component of the three-nucleon forces. These effects are numerically
investigated by the Faddeev calculation.
