We study the slave-boson $t\ensuremath{-}J$ model of cuprates with high superconducting transition temperatures, and derive its low-energy effective field theory for the charge-spin separated state in a self-consistent manner. The phase degrees of freedom of the mean field for hoppings of holons and spinons can be regarded as a U(1) gauge field,
Ai. The charge-spin separation occurs below a certain temperature,
TCSS, as a deconfinement phenomenon of the dynamics of
Ai. Below a certain temperature
TSG(<TCSS), the spin-gap phase develops as the Higgs phase of the gauge-field dynamics, and
Ai acquires a mass
mA. The effective field theory near
TSG takes the form of a Ginzburg-Landau theory of a complex scalar field $\ensuremath{\lambda}$ coupled with
Ai, where $\ensuremath{\lambda}$ represents d-wave pairings of spinons. Three dimensionality of the system is crucial to realize a phase transition at
TSG. By using this field theory, we calculate the dc resistivity $\ensuremath{\rho}.$ At
T>TSG, $\ensuremath{\rho}$ is proportional to T. At
T<TSG, it deviates downward from the T-linear behavior as $\ensuremath{\rho}\ensuremath{\propto}T{1\ensuremath{-}{c(T}_{\mathrm{SG}}\ensuremath{-}{T)}^{d}}.$ When the system is near (but not) two dimensional, due to the compactness of the phase of the field $\ensuremath{\lambda},$ the exponent d deviates from its mean-field value
1/2 and becomes a nonuniversal quantity which depends on temperature and doping. This significantly improves the comparison with the experimental data.