  General non-standard $tbW$ couplings are studied as model independently as
possible based on the effective Lagrangian consisting of the dimension-6
operators, which is an extension of the standard-model Lagrangian. The
$tbW$-interaction Lagrangian in this framework includes four kinds of
couplings, which could be complex. Constraints on those non-standard $tbW$
couplings are obtained by comparing the experimental data related to the $t\to
b W$ process with the corresponding theoretical formulas derived from the
effective Lagrangian. The constraints on some sets of the non-standard
couplings are shown not to be so strong because those couplings balance out
each other as we treat all the non-standard couplings as complex numbers at the
same time.
