A Sagnac-type fiber-optic gyroscope having a light source, a fiber coil consisting of an optical fiber, and a receiver arrangement which are all connected with a directional coupler in such a manner that wide-band light of the light source is fed into the two ends of the fiber coil and passes through it in opposite directions, the light emerging from the ends of the fiber coil and forming an interference signal in the directional coupler which is detected. An auxiliary coil is also connected with the directional coupler into which light from the single light source is also coupled, the dimensions of the auxiliary coil and of the main coil with respect to their length and their diameter being such that a measuring range is obtained that is expanded by the auxiliary coil and in which, for the whole measuring range, unambiguous measuring values may be determined for the rate of rotation of the fiber-optic gyroscope.