  Possible non-standard tuZ and tcZ interactions, which induce flavor-changing
neutral-current decays of the top quark, are studied in the
effective-Lagrangian framework. The corresponding Lagrangian consists of four
kinds of non-standard couplings coming from SU(3)xSU(2)xU(1) invariant
dimension-6 effective operators. The four coupling constants in each
interaction are treated as complex numbers independent of each other, and
constraints on them are derived by using the present experimental limits of the
branching fractions for t -> u Z and t -> c Z processes. Future improvements of
those constraints are also discussed as well as possibilities of measurements
of these couplings at the High-Luminosity Large Hadron Collider.
