A technique of measuring energy of electrons excited by exposing a semiconductor material to solar ray is proposed. A surface layer having a negative electron affinity is formed on the surface of a semiconductor material. The semiconductor material is placed in a vacuum environment and exposed to solar ray. Photoelectrons emitted from the surface layer having the negative electron affinity are guided to an energy analyzer, and the energy of electrons excited by the solar ray is measured. Since the surface layer having the negative electron affinity is used, the photoelectrons are obtained from the electrons excited by the solar ray, and thereby energy measurement becomes possible.