To allow a quick and reliable check with a simple structure as to whether a signal transmission medium has been inserted up to a predetermined position or whether a lock operation has been completed, an insulating housing is provided with lock checking device allowing a displaced state or an engaged state of a lock member to be visually checked. With this, the displaced state of the lock member or a state in which the signal transmission medium has fallen in an engagement positioning part when the signal transmission medium is inserted up to the predetermined position can be visually checked through the lock checking device, thereby allowing the quality of an insertion state of the signal transmission medium to be immediately determined.