PURPOSE: To accurately measure the force acting on a cantilever without adjusting the rotation of an optical position detector.

CONSTITUTION: A displacement measuring section 40 which detects the displacement of a cantilever 20 incorporates an optical position detector 42 which detects the displacement of the cantilever 20 as displacement components respectively parallel to two axes perpendicular each other on a surface to be detected from the positional change of the spot of reflected light L' from the surface to be detected and a signal processing circuit 43 which performs linear transformation corresponding to the coordinate transformation of the two axes perpendicular to each other into another two axes perpendicular to each other on the surface to be measured on a displacement component detected by means of the detector 42 and outputs the transformed component.