  It is demonstrated that an infinite set of string-tree level on-shell Ward
identities, which are valid to all sigma-model loop orders, can be
systematically constructed without referring to the string field theory. As
examples, bosonic massive scattering amplitudes are calculated explicitly up to
the second massive excited states. Ward identities satisfied by these
amplitudees are derived by using zero-norm states in the spetrum. In
particular, the inter-particle Ward identity generated by the D2xD2' zero-norm
state at the second massive level is demonstrated. The four physical
propagating states of this mass level are then shown to form a large gauge
multiplet. This result justifies our previous consideration on higher
inter-spin symmetry from the generalized worldsheet sigma-model point of view.
