The amplitude control of a cantilever based on the van der Pol model is performed through feedback using measurement data on a deflection of the cantilever. A self-oscillating circuit integrates a deflection angle signal of a cantilever detected by a deflection angle measuring mechanism using an integrator, multiplies a resulting integral value by linear feedback gain generated by a gain generator, and an output corresponding to the linear feedback signal is generated. Also, the self-oscillating circuit cubes the deflection angle signal using analog multipliers, integrates the resulting values using integrators, multiplies the resulting integral values by a nonlinear feedback gain generated by a gain generator, and an output corresponding to the nonlinear feedback signal is generated. Furthermore, the self-oscillating circuit adds the outputs together using an adder, and a voltage signal for a piezo element is generated.