PURPOSE: To improve the measurement precision by arranging a reference lattice having the specified period structure, and measuring the relative displacement between a photon scan tunneling microscope and the reference lattice.

CONSTITUTION: The diverged light from a light source 104 becomes the parallel light by a collimator lens 105, and is reflected or incident by a reflecting film 109 and a small opening 106. A part of the scattered light at the opening 106 is transmitted through a photo wave guide 103, and incident on a light detector 107. The light becomes the evanescent light at the opening 106 due to the quantum effect with extremely small amount of spill outside the film 109. Thus, when an object is present in the spilled range, the light and the object interact with each other, and when the object is displaced, the complex index of refraction in the vicinity of the opening is modulated, and the scattered light at the opening 106 is also modulated. Thus, the signal by the displacement can be obtained by using the reference lattice 108 whose pitch is known to the object, and the displacement of the lattice 108 or the detection optical system can be measured from the signal and the pitch size.