A releasable electric connector comprising: an insulating housing; and a releasable lock member provided inside the insulating housing and coupled thereto by means of a coupling support part, the releasable lock member holding, in an approximately unmovable state, a terminal portion of a signal transmission medium inserted in the insulating housing up to a predetermined position, the electric connector being configured so that, after a part of the lock member is elastically displaced so as to override the surface of the signal transmission medium with the insertion of the signal transmission medium, the part of the lock member falls in an engagement positioning part (Fa) provided at a terminal portion of the signal transmission medium to become in an engaged state, wherein the insulating housing is provided with lock checking device allowing a displaced state or an engaged state of the lock member to be visually checked, wherein the lock checking device is configured of a window part or a slit part provided so as to penetrate through a wall surface configuring the insulating housing, and through the slit part, the part of the lock member protrudes toward outside of the insulating housing when the lock member is elastically displaced, wherein the lock member is provided with a lock release pressing part to which an operating forces is added in a direction of elastically displacing the lock member, and so as to extend overlapping the lock release pressing part, a lock operation cover part is provided for the insulating house, and wherein the lock operation cover part is configured that the lock operation cover parts sandwiches the lock release pressing part from the forward and backward in the direction in which an operating down force is added so as to make the lock member displace elastically. an insulating housing; and a releasable lock member provided inside the insulating housing and coupled thereto by means of a coupling support part, the releasable lock member holding, in an approximately unmovable state, a terminal portion of a signal transmission medium inserted in the insulating housing up to a predetermined position, the electric connector being configured so that, after a part of the lock member is elastically displaced so as to override the surface of the signal transmission medium with the insertion of the signal transmission medium, the part of the lock member falls in an engagement positioning part (Fa) provided at a terminal portion of the signal transmission medium to become in an engaged state, wherein the insulating housing is provided with lock checking device allowing a displaced state or an engaged state of the lock member to be visually checked, wherein the lock checking device is configured of a window part or a slit part provided so as to penetrate through a wall surface configuring the insulating housing, and through the slit part, the part of the lock member protrudes toward outside of the insulating housing when the lock member is elastically displaced, wherein the lock member is provided with a lock release pressing part to which an operating forces is added in a direction of elastically displacing the lock member, and so as to extend overlapping the lock release pressing part, a lock operation cover part is provided for the insulating house, and wherein the lock operation cover part is configured that the lock operation cover parts sandwiches the lock release pressing part from the forward and backward in the direction in which an operating down force is added so as to make the lock member displace elastically.