PURPOSE: To obtain a wide-band optical transmission line by composing the optical transmission line of optical fibers which have center dips and optical fibers which have no center dip.

CONSTITUTION: The optical fibers M1WM5 having the center dips and the optical fibers V1WV5 having no center dip are connected together by welding in the array order of M1, V1, M2, V2, M3, V3, M4, V4, M5, and V5 to constitute the optical transmission line. A laser diode (LD) and an avalanche diode (APD) are connected to the incidence and projection ends respectively and a measurement of its band is 286MHz.km. It is recognized that the transmission line composed of optical fibers having the center dips and optical fibers having no center dip, has generally wide-band characteristics even when said line is a long-sized line, specially, when optical fibers with center dip and optical fibers having no center dip are connected alternately, mode conversion of low-order mode occurs at every connection part, so the band characteristics are more improved.