A rear lamp apparatus of a vehicle is provided. The apparatus includes a light source that emits light having a first wavelength band, a light filter disposed with a first side facing an exterior and a second side facing an interior, that reflects light having the first wavelength band, and transmits light having a second wavelength band that is different from the first wavelength band. A light diffraction element is disposed to receive light emitted from the light source and changes a transmission path of incident light to direct. The light emitted is transmitted to the first side of the light filter by the light diffraction element and is reflected to be directed toward the exterior of the vehicle to prevent the transmission to the interior of the vehicle. Accordingly, a rear lamp is turned on at a side of rear glass and exterior and interior visual fields are improved.