We have developed an algorithm for correcting Fourier-transform infrared spectra suffered by photoconductive MCT detector nonlinearity. We assumed that interferogram data exceeding a certain intensity level are suffered from the influence of the nonlinearity in a form of exponential function. The nonlinear model is expressed by using two parameters to carry out the nonlinear optimization procedure. A criterion for the optimization is to minimizing the so called non-physical-energy to be zero, which appears in a low wavenumber region where the detector has no response. In the course of correction, we found out that the frequency response of an ac coupling circuit between the detector and a preamplifier should be taken into account. Calibration curves for real samples, in comparison with those obtained from an MCT detector attached with a linearizer circuit, indicates that the proposed algorithm works fairly well.