Intensity of near-ultraviolet light or visible light of 180 to 700 nm emitted from a solid sample, such as an organic semiconductor, irradiated with an electron beam is measured, while kinetic energy (accelerating energy) of the electron beam is changed in a range of 0 to 5 eV so as to obtain a spectrum. Peaks are detected from the spectrum, and the energy thereof is defined as unoccupied-states energy of the sample. The onset energy of the first peak represents electronic affinity energy (electron affinity) of the sample. Since the energy of the electron beam irradiated onto the sample is 5 eV or less, almost no damage is exerted on the sample even when the sample is an organic semiconductor.