PURPOSE: To improve the capability of controlling the access of probe to a specimen by assembling an optical lever arm, a probe and the optical system observing the specimen and facilitate fixing and treatment of the probe assembly by magnetically fixing a mount for the probe.

CONSTITUTION: By fixing a scanning means 20, observing means 110 and a photodetector 88 to the main body, magnetically fixing an optical lever arm 40 and providing the base with a gap for containing a specimen 18, a scanning force microscope is assembled. Laser light from a laser source 70 is casted on the specimen 18, scanned with a scanning means 20 in X and Y directions, and deflected with a reflex cantilever 46. The photodetector 88 generates electric signal in response to the deflected laser light and send it to a feedback control means 64. The control means 64 drives an electric drive means 104 to move a support stage 24 up and down in Z direction and hold the force added to the probe tip constant. In this period, the access of probe assembly to the specimen 18 is monitored with the observing means 110 to ensure safe and exact positioning.