In a tip portion structure basically having a substrate, a plate spring, and a ground block, the substrate is attached to a signal line on a back surface of the substrate and is contacted on the tip with the signal electrode of the DUT placed on a device stage. The plate spring is made of a resilient material, placed on the front side of the substrate, and positioned to apply a pressure to the substrate. The ground block is positioned between the signal line and the device stage functioned as a ground electrode of the DUT. Alternatively, the tip portion structure further may have a ground plate or a ground surface formed of a conductive thin plate covering entirely the front surface of the substrate, and shaped to surround the signal line in cooperation with the ground block. A plurality of the signal lines may be arranged in parallel on the same plane of the substrate. Another tip portion structure is based on a coaxial cable to be cut from the center at a plane perpendicular to the axial direction thereof along one or more oblique plane. A metal ring fitted over a periphery of the coaxial outer conductor may be used.