PURPOSE:To provide ultrasonic oscillation detecting method and sample observing method for interatomic force microscope in which the viscoelaslic properties and the like can be measured at a frequency higher than 1 MHz by generating ultrasonic oscillation of 1 MHz or above In a sample and detectng the ultrasonlc oscillation using a common cantilever. CONSTITlUTION:A sample 8 is subjected to ultrasonic oscillation at a frequency sufficiently higher than the resonance frequency of a cantilever 11. Since the force functioning between a probe 4 and the sample 8 exibit nonlinear dependency on the distance, a displacement depending on the amplitude of oscillation Is Induced In the cantilever 11. Ultrasonic wave is detected by measuring the displacement.