Although monitoring the wavelength stability of a free-electron laser provides users valuable useful information, a single-shot measurement of the laser spectrum is not always an easy task in the mid/far-infrared wavelength region due to the lack or very high cost of an array-type photodetector, etc. We propose to use fringe-resolved autocorrelation to monitor the wavelength stability of an oscillator-type free-electron laser. The idea is verified by the numerical experiments for intensity and fringe-resolved autocorrelations which include shot-to-shot fluctuations of intensity, duration, and central wavelength of micropulses as well as the chirps of both micro/macro pulses. (C) 2012 The Japan Society of Applied Physics