A scanning tunnel microscope includes a piezo-actuator (16) movable in a direction Z perpendicular to the surface of a sample (12) and provided with a scanning probe (14) projected downward from the bottom thereof. The scanning probe (14) is moved in the direction Z by the piezo-actuator (16). The piezo-actuator (16) includes a position measuring probe (22) projected therefrom, parallel to the surface of the sample (12), from the lower end of one side thereof. A reference sample (26) having concaves formed on a surface thereof at predetermined intervals is fixed to the piezo-actuator (16). When the scanning probe (14) is moved in the direction Z, the position measuring probe (22) is moved along the reference sample (26), so that tunnel current having a waveform which reproduces the shape of the concaved surface of the reference sample (26) is detected. Thus, the moving distance of the scanning probe (14) can be obtained from the waveform of tunnel current detected.