PURPOSE: To simplify the adjusting method, by providing a means changing an inner diameter of an outer conductor of a signal transmission line, adjusting the distance between the outer and the inner conductors with this means for the purpose of impedance matching, for attaining the impedance matching over a wide range.

CONSTITUTION: A matching device is provided with high frequency signal connectors 1,2, a high frequency signal is applied from the connector 1 or 2, and transmitted to the other connector 2 or 1 through a signal transmitting conductor 3 at the center thereof. The matching device is provided with two insulators 4 movable externally and having a prescribed length therein so as to form its center through which the conductor 3 having an impedance matched to the characteristic impedance of the line passes. A component varying the inner diameter of the outer conductor 6 is provided at the center in the signal transmitting direction, one end of four inner diameter varying sections 5-2 of the component varying the inner diameter is fixed to a moving section 5-1, the other end is pushed with a retaining spring 5-3 for the inner diameter varying section. Further, the outer conductor 6 is provided with projections 5-4 for the inner diameter varying section adjusted to the thickness of the varying sections 5-2 for varying the inner diameter of the outer conductor 6.