  The temperature-dependent infrared properties of
$\theta$-(BEDT-TTF)$_2$RbZn(SCN)$_4$ were investigated. In the temperatures
above the metal-insulator (M-I) transition, the optical conductivity remains
finite in the low energy region, but shows no conventional Drude feature
typical of metals. Below the M-I transition temperature, the low-energy
spectral weight is significantly reduced. An opening of optical gap at about
300 cm$^{-1}$ is observed clearly in the polarization parallel to the
donor-stacking direction. Analysis of both the electronic and vibronic spectra
suggests a redistribution of charges in ET molecules as temperature decreases
across the M-I transition, providing strong support for the charge-ordering
state at low temperatures.
