Disclosed is a three-wavelength optical multiplexer which is compact, and which multiplexes light having different wavelength incident to three single-mode optical fibers, particularly light of red, green, and blue at transmittance above a certain reference. Specifically disclosed is a three-wavelength optical multiplexer (100) which comprises three single mode optical fibers (1, 2, 3), wherein the three single-mode optical fibers (1, 2, 3) having incidence/emission ports are arranged in parallel to one another in this order as observed from the cross sectional direction orthogonal to the fiber axis direction, and are fused and drawn, and wherein the relationship of a secondary-mode cutoff wavelength (C1) which is one of the parameters of the single mode optical fiber (1) and the single-mode cutoff wavelength (2), a secondary-mode cutoff wavelength (C2) which is one of the parameters of the single-mode optical fiber (3), and the wavelength (λ3) of light incident from the single-mode optical fiber (3) is sin C1