An optical data transmission apparatus is provided, in which a first communication unit 101 and a second communication unit 102 arranged to freely move relatively to each other each include an optical signal transmission unit having a laser diode 120 for emitting single-mode light, a multi-mode optical fiber 111 for guiding a single-mode optical signal output from the laser diode 120 , converting the single-mode optical signal into a multi-mode optical signal, and outputting the multi-mode optical signal, an optical lens 112 for forming the optical signal output from the multi-mode optical fiber 111 into parallel light, and a first polarization element for passing the optical signal polarized in a predetermined direction out of optical signals output from the optical lens 112.