Metal pipes containing optical fibers are connected to each other directly or indirectly by use of a sleeve. In case of using the sleeve, the sleeve is connected at one end to one of the metal pipes, after the optical fibers are connected. Then, the sleeve is elongated by a predetermined length, so that the sleeve is connected at the other end to the other metal pipe. Consequently, no tensile stress resides in the connected optical fibers in the connected structure. When the metal pipes are connected directly to each other, one or both of the metal pipes are elongated, after the optical fibers are connected. Then, the metal pipes are connected by use of the elongated portions. Consequently, the same result is obtained as using the sleeve.