  The process of top-quark pair production followed by semileptonic decays at
future high-energy e^+e^- linear colliders is investigated as a possible test
of physics beyond the Standard Model. Assuming the most general non-standard
forms for gamma t t-bar, Z t t-bar and Wtb couplings, the energy spectrum of
the single lepton l^{+-} and the energy correlation of l^+ and l^- emerging
from the process e^+e^- --> t t-bar --> l^{+-} ... /l^+l^- ... are calculated.
Expected precision of the non-standard-parameter determination is estimated
adopting the recently-proposed optimal method.
