A signal transmission medium inserted into an insulating housing can be held and released in a favorable manner with a simple structure. A pair of lock release parts are arranged to face opposite each other at both outer ends of a signal transmission medium, this pair of lock release parts being integrally and continuously formed to release arms integrally extending from the insulating housing to be movable to approach and separate from each other, with a lock release link mechanism being provided for causing the locking portion to displace in an unlocking direction by moving the pair of lock release parts in directions approaching each other.