To provide a built-in type composite transmission line processing device eliminating increase in an occupation area of a cable drum on a deck, and retaining a safety and highly reliable electric and optical connections in spite of the rotation and the rotation speed of the cable drum.

This built-in type composite transmission line processing device is provided with a fixed drum shaft 15 penetrating through a hollow part 32A in the inside of a winding drum 32 of the cable drum 2, a driven sub-drum 17 rotatably journalled to the drum shaft 15 in the hollow part 32A integrally with the cable drum 2, a stationary sub-drum 18 fixedly disposed in the drum shaft 15 adjoining to the driven sub-drum 17. The continuous part of an optical transmission conductor 10B wound or rewound by the driven sub-drum 17 is wound or rewound through the mediation of the stationary sub-drum 18 by the turning of a cable traverser 20 turning along the outer circumferences of the driven sub-drum 17 and the stationary sub-drum 18.