Incident light 19 emitted from a laser light source 18 is reflected on an upper surface of a cantilever 13, so that reflected light 19a enters light detection means 20. Since the incident light 19 and the reflected light 19a are in a plane not including a long axis of the cantilever 13, movements of the reflected light 19a due to change in a deflection amount θ of the cantilever 13 and due to change in a fine vertical movement amount z thereof are different in direction on the light detection means 20. This enables the change in the deflection amount θ of the cantilever 13 and the change in the fine vertical movement amount z thereof to be separated from output of the light detection means 20.