A method for detecting a period of a periodically varying signal and/or an inclination of a specimen utilizing an exposure apparatus wherein a coherent light is divided into first and second lights and the first light is irradiated onto the specimen at a predetermined angle, and a reflected light thereof and a reference light as the second light interfere with each other so as to form interference fringes, the interference fringes are detected, and the inclination of the specimen is detected from a pitch representing a period of the interference fringes. The method includes detecting a spectrum of a signal intensity obtained from the detected interference fringes or the periodically varying signal, subjecting the detected spectrum data to a fast complex Fourier transformation, calculating a true spectrum peak position j.sub.R in accordance with a relationship j.sub.R =j.sub.0 +.DELTA., where j.sub.0 is a detected spectrum peak position and .DELTA. is a correcting value estimated from the Fourier transformed spectrum data adjacent to j.sub.0, and calculating the pitch P of the period of the signal intensity information in accordance with a relationship P=N/j.sub.R, where N is the number of sample data points for the complex Fourier transformation. Further, the method includes calculating the inclination of the specimen from the pitch P.