The present invention relates to a method of forming a transmission line capable of measuring more precise connection losses at low cost, and so on. At least of first and second optical fibers to be connected as components of the optical transmission line is selected such that, at one wavelength .lamda. contained in the wavelength range of 1260 nm to 1625 nm, predetermined relationships defined by the Rayleigh scattering coefficients of the first and second optical fibers, the mode field diameters of the first and second optical fibers at the wavelength .lamda., and the transmission losses of the first and second optical fibers at the wavelength .lamda. can be satisfied between the first and second optical fibers.