A probe for scanning a sample is attached to a probe electrode supported by a cylindrical piezoelectric actuator. The actuator has four drive electrodes on its periphery, and deforms in the three axial directions in accordance with the voltage applied to the drive electrodes. A girdling electrode is provided between the actuator and the probe electrode. The girdling electrode is insulated from the probe electrode and the drive electrodes by insulator members provided on its upper and lower surfaces. A bias voltage signal S1 is input to an operational amplifier of which the output is connected to the girdling electrode. The amplifier is a voltage follower for equalizing the potential of the girdling electrode to that of the bias voltage signal S1. An operational amplifier has a non-inversion input to which the bias voltage signal S1 is input, and an inversion input connected to the probe electrode. While equalizing the potential of the probe electrode to that of the bias input voltage S1, the amplifier converts a tunnel current flowing between the probe and the sample to a voltage signal S2. An operational amplifier subtracts a bias voltage component from the voltage signal S2, and outputs a tunnel current signal S3.