A transmission apparatus with flexible structure for introducing external cables that controls signal transmission efficiently. A cable connector for introducing a cable from the outside is fixed on a sub-printed circuit board. A main printed circuit board has a guide rail along which the sub-printed circuit board can be inserted reversely to change the direction from which the cable is introduced at the time of the sub-printed circuit board being housed. An enclosure has ducts for bunching cables. Transmission units are mounted in the enclosure.