A fiber-optic gyroscope has a polarizer, a fiber coil and couplers. A light beam is polarized, divided into two beams and introduced into the fiber coil. Two depolarizers are provided; one between a light source and a polarizer, the other between the fiber coil and a coupler. To clarify the condition for avoiding interference between two depolarizers is a purpose of this invention. Four birefringent materials A, B C and D should satisfy inequalities;.vertline.P.sub.a B.sub.a L.sub.a +P.sub.b B.sub.b L.sub.b +P.sub.o B.sub.o L.sub.o +P.sub.d B.sub.d L.sub.d .vertline.-B.sub.5 L.sub.5 >Cwhere P.sub.a, P.sub.b, P.sub.o, and P.sub.d are triplet factors which take one of -1, 0 or +1 independently, B.sub.a, B.sub.b, B.sub.o and B.sub.d are birefringence and L.sub.a, L.sub.b, L.sub.o and L.sub.d are lengths of A, B, D and D. B.sub.5 L.sub.5 is extra birefringency of the fiber coil or couplers.