A current sensor which is installed to a conductor flowing a current and measures the current flowing through the conductor, the current sensor including an optical circulator, a birefringence element, a Faraday rotator, an optical fibre for the current sensor and a mirror (1, 10), wherein the Faraday rotator is installed at the one end of the optical fibre for the current sensor and the mirror (1, 10) is installed at the other end thereof, characterized in that the mirror (1, 10) is an optical fibre birefringence compensation mirror (1, 10), the optical fibre birefringence compensation mirror (1, 10) comprising: an optical fibre; a first birefringent element; a lens; a mirror, wherein the optical fibre of the optical fibre birefringence compensation mirror (1, 10) is optically connected to the optical fibre for the current sensor and wherein the optical fibre birefringence compensation mirror (1, 10) further comprises a magnet and a Faraday rotator which is applied with a magnetic field from the magnet to be magnetically saturated and has a Faraday rotation angle of 45 degrees, wherein components of the birefringent element, the Faraday rotator, and the mirror are disposed in the order of the birefringent element, the Faraday rotator, and the mirror from a light incidence/emission end surface of the optical fibre, the optical fibre is of a single mode type, a light beam propagating through the optical fibre is divided into two perpendicular linearly polarized light beams (8a, 8b) of a ordinary ray and an extraordinary ray by the birefringent element, the two perpendicular linearly polarized light beams (8a, 8b) are condensed by the lens, the two linearly polarized light beams (8a, 8b) transmit through the Faraday rotator, therefore polarization planes thereof are rotated by 45 degrees, and the two linearly polarized light beams (8a, 8b) are reflected at one point on a surface of the mirror in point symmetry, the two reflected linearly polarized light beams (8a, 8b) re-transmit through the Faraday rotator, therefore the polarization planes of the two linearly polarized light beams (8a, 8b) are further rotated by 45 degrees, the two linearly polarized light beams (8a, 8b) are incident on the birefringent element again, consequently one light beam is re-combined, the re-combined light beam is incident on the optical fibre, and a shift amount of the extraordinary ray in the birefringent element is equal to or larger than twice a mode field diameter of the optical fibre. an optical fibre; a first birefringent element; a lens; a mirror, wherein the optical fibre of the optical fibre birefringence compensation mirror (1, 10) is optically connected to the optical fibre for the current sensor and wherein the optical fibre birefringence compensation mirror (1, 10) further comprises a magnet and a Faraday rotator which is applied with a magnetic field from the magnet to be magnetically saturated and has a Faraday rotation angle of 45 degrees, wherein components of the birefringent element, the Faraday rotator, and the mirror are disposed in the order of the birefringent element, the Faraday rotator, and the mirror from a light incidence/emission end surface of the optical fibre, the optical fibre is of a single mode type, a light beam propagating through the optical fibre is divided into two perpendicular linearly polarized light beams (8a, 8b) of a ordinary ray and an extraordinary ray by the birefringent element, the two perpendicular linearly polarized light beams (8a, 8b) are condensed by the lens, the two linearly polarized light beams (8a, 8b) transmit through the Faraday rotator, therefore polarization planes thereof are rotated by 45 degrees, and the two linearly polarized light beams (8a, 8b) are reflected at one point on a surface of the mirror in point symmetry, the two reflected linearly polarized light beams (8a, 8b) re-transmit through the Faraday rotator, therefore the polarization planes of the two linearly polarized light beams (8a, 8b) are further rotated by 45 degrees, the two linearly polarized light beams (8a, 8b) are incident on the birefringent element again, consequently one light beam is re-combined, the re-combined light beam is incident on the optical fibre, and a shift amount of the extraordinary ray in the birefringent element is equal to or larger than twice a mode field diameter of the optical fibre.