Good signal transmission characteristics can be stably obtained by a simple configuration. A void portion formed by inner wall surfaces of a groove disposed so as to face a wire connection part of a cable-shaped signal transmission medium and an inner conductor contact is provided in part of an insulative pressing plate, which is opened/closed integrally with a shell cover portion, and the wire connection part thereof and the void portion are disposed in a radial-direction inner region of a shield shell. As a result, an air layer for impedance adjustment formed by the void portion of the insulative pressing plate is disposed so as to face the wire connection part, at which mismatching of impedance easily occurs, and the matching degree (VSWR) of impedance can be efficiently adjusted.