PURPOSE: To enable the measurement of the concentration of an excessive carrier of a half-insulating semiconductor at a room temperature free from the presence of free exciton emission by measuring changes in specific resistance of a half-insulating semiconductor with a specific resistance value exceeding a specified value by light irradiation.

CONSTITUTION: A sample of a half-insulating semiconductor, for example, a half- insulating GaAs and a half-insulating CdTe with a specific resistance of above 1×107Ωcm and with an electron mobility μ and a hole mobility μp known is formed in a rectangle desirably with a size of about 5mm×5mm and electrodes are formed at four tops of the rectangle to facilitate the measurement of changes in specific resistance. As the concentrations Δnp and Δpp of an excessive carrier generated by the irradiation of light is given by Δnp=Δpp as electrons and holes are generated in a pair and is determined by Δnp=4pV/[πD2λq(μn+μp)]. (wherein : electric conductivity in light irradiation, q: electric element, V: sample volume, D: beam diameter of irradiation light, λ: depth of irradiation light). This eliminates free exciton emission thereby enabling comparison of the intensity of emission between samples different in surface condition.