PURPOSE: To improve pressure-resistance strength and airtightness, make connection and assembling easy and reduce size and weight by providing rod lenses, a repeater electric circuit and photo-electric transducers.

CONSTITUTION: Rod lenses 2 are connected to the end part of opticalfibers 1. The lenses 2 are made of sapphire or the like and are the lenses having refractive index distributions wherein refractive indices reduce nearly square from the central axis toward the periphery. An end face plate 3 is assembled hermetically to a pressure cylinder 4 through welding or the like. One or a number of the lenses 2 are fixed to the end face plate 3 from its back by means of fixing fittings 5 and push nuts 6. Photo-electric transducers 7 are fixed to the fixing fittings in the positions having a slight clearance from the lenses 2 and are connected to a repeater electric circuit 9 by means of metal wires 8. The light signals having been transmitted radiate the luminous fluxes which do not radiate by the lenses 2 and these luminous fluxes are inputted to the photo- electric transducers 7 and are thereby converted to electric signals which are then transmitted to the repeater electric circuit 9.