A light combining apparatus using a flat plate operating to combine at least two incident light beams entering the flat plate at a first angle with respect to the vertical direction on an incidence plane with wavelengths different from each other. The apparatus comprises a first anti-reflection coating film for allowing only a light beam with a first wavelength to penetrate into the flat plate without reflection, a first wide band high-reflection mirror for reflecting the light beam with the first wavelength which penetrates through the first anti-reflection coating film at a second angle which is different from the first angle, a first dichroic mirror for generating a first combined light beam by allowing only a light beam with a second wavelength different from the first wavelength and reflecting the light beams with other wavelengths and by reflecting the reflected light beam with the first wavelength at the second angle, and for combining the light beam with the second wavelength and the reflected light beam with the first wavelength, a second wide band anti-reflection coating film for allowing the combined light beam to penetrate without reflection; and a material within the flat plate through which the light beam penetrated through the first anti-reflection coating film and the first dichroic mirror and the light beam reflected from the first wide band high-reflection coating film and the first dichroic mirror pass.