A scanning probe is microscope has a cantilever having a probe at a disal end thereof and an oscillator for generating a resonance signal near a resonance of the cantilever. A vibrating device receives the resonance signal as a driving signal for vibrating the cantilever. A variable gain amplifier adjusts a gain of displacement signal corresponding to displacement of the vibrating cantilever so as to satisfy the equation G=(A/A₀)*G₀ to control a quality factor value of the cantilever resonance to an optimal quality factor value, where G represents a gain value of the variable gain amplifer, A represents a preselected oscillation amplitude of the oscillator, A₀ represents an initial oscillation amplitude of the oscillator, and G₀ represents a gain value of the variable gain amplifier when the initial oscillation amplitude of the oscillator is A₀.