The invention relates to an optical waveguide-type optical multiplexer, an optical waveguide-type multiplexing light source device and an image projection device, where the intensity of a light beam emitted from a light source is attenuated to a desired value without installing an additional optical attenuator element. One of a plurality of optical waveguides on the light emission side for emitting light distributed/multiplexed in an optical multiplexer unit excluding an optical waveguide on the light emission side in which the greatest output light power can be gained for each wavelength from among the optical waveguides on the light emission side in the case where a plurality of light sources is driven is used as an optical waveguide for light emission.