To provide a method in which the tertiary nonlinear susceptibility rate of a powder sample or a film sample is measured and to provide its apparatus.

A rotating base 41 is turned, and a photodiode 40 is retreated to a position which is not disturbing. In succession, a shutter 3 is opened, and a sample is irradiated with a beam 32 and a beam 33. The angle of incidence of the beam 33 is set to be smaller than a total-reflection critical angle. A rotating base 10 is turned, an angle is swept at about 62°C, and an angle at which a signal intensity becomes strongest is searched. At this time, the magnitude of the angle of incidence of a beam 31 is made just equal to that of the angle of incidence of the beam 32m the angle of the rotating base at this time is read out, and a precise angle of incidence is found. On the basis of the angle, a tertiary nonlinear susceptibility rate is found by a computer 16. Consequently, the tertiary nonlinear susceptibility rate of a powder sample or a nonhomogeneous film sample which is hard to measure can be measured.