A time resolved, nonlinear complex susceptibility measuring apparatus (1) that is capable of measurement unaffected by any distortion of the wave front of a probe light whereby a temporal change in the nonlinear complex susceptibility of a nonlinear optical material that occurs when it is irradiated with a light pulse in a femtosecond range is measured using a pair of polarized lights orthogonal to each other which are formed by splitting a single light pulse into a reference and a probe light (5) and (6) in a polarized light splitting Sagnac type interference light path (8). A direction of polarization converting mechanism for rotating a direction of polarization of the reference and probe lights by an angle of 90 DEG in the polarized light splitting Sagnac type interference light path is included to align the directions of polarization on a test specimen (3). A phase difference between the reference and probe lights which are output from the polarized light splitting Sagnac type interference light path is swept by a phase difference sweep mechanism (9) whereby a time resolved, nonlinear complex susceptibility is found from a phase difference sweep interference waveform obtained by measuring the intensity of interference light between the reference and probe lights for each of such phase differences swept. <IMAGE>