A light combining apparatus using two flat plates combines three separate light beams having blue, green and red wavelengths to a single combined light beam for a light source by an easier and simple small-sized device and makes the combined light beam smoothly enter the place for implementing the final information. The apparatus includes a reflection mirror for reflecting light beams with all of the wavelengths, at least one filter for transmitting only at least one light beam with a specific wavelength and reflecting light beams with the other wavelengths, and a medium located between the reflection mirror and the filter, through which the incident light beams and the reflected light beams pass, wherein the three light beams are combined into a single light beam within the medium which single light beam then exits the apparatus. A method of combining separate light beams having different wavelengths is also disclosed.