In order to examine the feasibility of realizing the scanning atom probe, a trial instrument was constructed by modifying a low-temperature ultrahigh vacuum scanning tunneling microscope (STM). A scanning electron microscope was mounted on the trial instrument to inspect the relative position of both the open hole at the end of the funnel-shaped extraction electrode and a cusp apex on a specimen surface. In the preliminary experiment, a single sharp W tip of the STM and two extraction electrodes with respective open hole diameters of 10 and 20 mu m were employed. The tip was brought near the electrode by employing a microscrew and a piezoactuator as with a conventional STM. The variation of the field emission current with the position of the center of the hole relative to the cusp apex was found to be significant enough to bring the apex precisely to the center of the hole. An interesting finding was that the field emission was observed from a tip with a radius of more than 2 mu m at a relatively low voltage, indicating that the field confined to a narrow space between the electrode and the tip apex was high enough to induce field emission.