A portion of light emitted from a laser source (11) for detecting a displacement of a cantilever (4) is extracted by a half mirror (20) and guided onto a photodetector (21) having a light-receiving surface divided into four sections. When the direction of the emitted light is inclined due to a change in the ambient temperature or other factors, the light spot formed on the light-receiving surface of the photodetector (21) moves. Accordingly, the amount and direction of the inclination of the emission direction can be recognized from the amount and direction of the movement of the light spot. A drive amount calculator (22) calculates a drive amount according to the amount and direction of the inclination, and operates an actuator (23) to rotate the laser source (11) around each of the Y and Z axes. This operation compensates for the inclination of the direction of the emitted light and thereby prevents the inclination from being falsely recognized as an irregularity on the sample surface.