Patent ID: 7975315

Claim:
An atomic force microscope that causes a cantilever to self-excitedly oscillate at a resonant frequency and obtains information on a sample on the basis of a resonant frequency shift caused by an interaction between the cantilever and the sample, comprising: a displacement sensor for detecting displacement of the cantilever; and a resonant frequency shift detecting section for detecting the resonant frequency shift caused by the interaction between the cantilever and the sample on the basis of a detection signal from the displacement sensor, wherein the resonant frequency shift detecting section including: a reference signal extracting section for extracting a reference signal from the detection signal from the displacement sensor, the reference signal having a cycle corresponding to oscillation of the cantilever and being restrained from a phase change in accordance with the resonant frequency shift in the cantilever; a phase shift signal extracting section for extracting a phase shift signal from the detection signal from the displacement sensor, the phase shift signal having a cycle corresponding to oscillation of the cantilever and having a phase shifted in accordance with the resonant frequency shift in the cantilever; and a phase detecting section for determining the resonant frequency shift by determining, on the basis of the reference signal and the phase shift signal extracted by the reference signal extracting section and the phase shift signal extracting section, a phase difference of the phase shift signal from the reference signal.