For easy removal of a flat-plate-shaped signal transmission medium while reduction in height and size is achieved with a simple structure, a structure is adopted for an electric connector as follows. That is, a release operating part of a lock releasing member which causes a lock member in an engaged state to make a transition to a released state is arranged along an insertion opening. The release operating part and a release acting part are arranged in opposite regions in a radius direction of rotation of a support shaft part. The release operating part can make reciprocating rotations between an initial position away from the insertion opening and an acting position close to the insertion opening. Thus, when a flat-plate-shaped signal transmission medium is removed, the flat-plate-shaped signal transmission medium can be held while the engaged state of the lock member with respect to the flat-plate-shaped signal transmission medium is released by one hand of an operator.