The oscillating behavior of a micro-cantilever probe plays a central role in the atomic force microscope for studying a nanoscale sample. The oscillatory phenomena in the microscope are numerically investigated by exciting the probe with a single frequency. We observe the non-resonant frequency components, which correspond to a frequency of transient beats superimposed on the stable solution, around the natural frequency of the probe, when the probe is close to the sample. The difference between the non-resonant frequency and the natural frequency changes when the tip-sample distance decreases. Furthermore, we investigate the originating point of the non-resonant frequency components as a function of the tip-sample distance. In addition, we perform an actual experiment for observing the frequency components near the resonant frequency.