An object of the invention is to provide an optical multiplexing method and an optical multiplexer, capable of fixing lights of different wavelengths at required wavelengths to stably multiplex them with a simple optical circuit configuration. To this end, in the present optical multiplexer, there is provided a tilted FBG, which is formed on an optical fiber through which a first light is propagated, with a grating direction thereof being tilted to an axial direction of the optical fiber, and has the sufficiently high reflectance to a second light (multiplexed light) of a wavelength different from that of the first light. The multiplexed light emitted from a light source is irradiated to the tilted FBG, via a free space from an angle direction determined according to a grating pitch of the tilted FBG and the wavelength of the multiplexed light, to be coupled within the optical fiber. Further, a low reflective FBG reflecting light coupled within the optical fiber is formed on the optical fiber, to achieve a resonance structure between the low reflective FBG and the light source, thereby stabling the wavelength of the multiplexed light.