PURPOSE: To achieve an increase in spatial resolutions by placing a semi- spherical prism transparent with a high refractive index in an infrared area on the top surface of a sample with the undersurface (flat surface) thereof tight thereon.

CONSTITUTION: Infrared rays emitted from a light source 2 pass through a Cassegrainian optical system 6 and enters a sample forming a spot at the center C of a semi-spherical prism 3. As a hemisphere 3 comprising Ge has a very high refractive index (4.0), the diameter of the spot is a quarter of that when the semisphere 3 is not used. As the refractive index of the hemisphere 3 is higher than that of the sample 1, when an incident light exists with an angle of incidence larger than a critical angle determined by both the refractive indexes, it is reflected totally on the undersurface of the hemisphere. Here, an evanescent field is formed in a short visual field on the side of the sample 1. Output light in the evanescent field via an interaction with the sample in the spot passes through the hemisphere 3 to form an image at the position of a pinhole 7. This enables realizing of spatial resolutions less than the wavelength of infrared rays used by photoelectric detection 8 of the image formation light.