  We calculate the spin-gap effect on dc resistivity in the t-J model of
high-$T_{\rm c}$ cuprates by using the Ginzburg-Landau theory coupled with a
gauge field as its effective field theory to get $\rho(T) \propto T \{1-c\:(T^*
-T)^d \}$, where $T^*$ is the spin-gap onset temperature. By taking the
compactness of massive gauge field into account, the exponent $d$ deviates from
its mean-field value 1/2 and becomes a nonuniversal $T$-dependent quantity,
which improves the correspondence with the experiments.
