In a two-beam interferometer, the moving speed of a movable mirror for producing interference light to generate a spectrum is changed at each completion of an operation cycle during which a reciprocal motion or one-way motion of the movable mirror is performed one or more times. Intensity signals of the interference light are obtained by a plurality of detection operations, where each detecting operation includes a series of detecting operations corresponding to the reciprocal motion or one-way motion of the movable mirror performed one or more times. A piece of information which changes depending on the moving speed of the movable mirror is extracted from the intensity signals obtained by the detection operations. This speed-dependent information corresponds to periodic noise superposed on the intensity signals. Accordingly, a false peak which would otherwise appear on the spectrum due to the periodic noise can be eliminated by removing the speed-dependent information.