To provide a technology to accurately calculate intensity per unit area of light exposed on a measurement area of a sample surface.

A light intensity measuring unit 1 for measuring intensity of light emitted from a microscope includes: an aperture diaphragm 3; a field diaphragm 5; at least one measuring lens 4 arranged between the aperture diaphragm 3 and the field diaphragm 5; and an interface section 2a for being mounted to the microscope. The aperture diaphragm 3 is located in the vicinity of a focal point on the rear side of the at least one measuring lens 4 so as to act like a pupil of an objective lens. The field diaphragm 5 is located in the vicinity of a focal point on the front side of the at least one measuring lens 4 so as to define an area used for calculation of the light intensity per unit area.