In a Michelson interferometer, two plane mirrors of the mirror unit (50) are embodied as two plane mirrors (51, 52) of different sizes, disposed eccentrically and parallel to one another. A hollow shaft (63), rotatably supported in a bearing block (62) and driven by a drive unit (65), is also provided, which is disposed between a beam splitter and the mirror unit (50) and whose axis of rotation (61) coincides with the axis of rotation of the mirror unit (50). The mirror unit (50) is secured to the hollow shaft (63) in such a way that the axis of rotation (61) of the hollow shaft passes through the mirror face of the smaller plane mirror (51), whose axis of rotation forms an acute angle (.beta.) with the vertical to the smaller plane mirror (51), so that the two beams formed at the beam splitter (1) pass, via the tilted mirrors, through the hollow shaft (63) to strike the centrally disposed mirror face of the smaller plane mirror ( 51), are reflected by it to the opposite mirror face of the larger plane mirror (52) and from there to two stationary plane mirrors (71, 72) in such a way that they strike the stationary plane mirrors (71, 72) vertically, and--reflected by those mirrors--take the same path in the opposite direction, recombine at the beam splitter, and are focused by means of an optical element onto a detector (4).