In an optical coupler in which optical fibers with GRIN lenses, each having an optical fiber and a GRIN lens joined coaxially, are disposed to face each other coaxially, with a preset inter-lens distance WD being provided, and to couple transmission light of one of the optical fibers with GRIN lenses to the other one of the optical fibers with GRIN lens, an intermediate position of the inter-lens distance WD is set to match a beam waist position of light with a shortest wavelength λm out of the transmission light, and a lens length z along an optical axis direction of the GRIN lens is set in accordance with a bottom of a coupling loss due to a distance-shift D of a beam waist position of light with a wavelength λi different from the λm out of the transmission light.
