To achieve a measurement of a defect density at a high accuracy even when a semiconductor crystal to be measured is thin.

A half-insulating GaAs single crystal is irradiated with exciting light having energy under a band gap energy (specifically, energy within a range of 1.40eV-1.47eV) while the intensity of the luminescence light emitted from the GaAs crystal associated with a two-stage excitation process phenomenon of the GaAs crystal is measured to accomplish the measurement of an EL2 density as proper defect density in the GaAs crystal. This measuring method does not employ the conventional lattice defect density measuring method by light absorption. This enables the measurement of a defect density at a high accuracy even when the semiconductor crystal to be measured is thin.