We investigated the temperature-dependent infrared properties ofthe compound θ-(BEDT-TTF)2RbZn(SCN)4. Attemperatures above the metal-insulator transition, theoptical conductivity remains finite in the low-energy region, butshows no conventional Drude feature typical of metals. Below themetal-insulator transition temperature, the low-energy spectral weight issignificantly reduced. The opening of an optical gap at about300 cm-1 is observed clearly in polarization parallel to thedonor stacking direction. Analysis of both the electronic andvibronic spectra suggests a redistribution of charges in BEDT-TTFmolecules as temperature decreases across the metal-insulator transition,providing strong support for a charge-ordering state at low temperatures.