An electric current measuring apparatus (1, 28) at least comprising: a light incident and emitting unit, a optical fiber for a sensor, a Faraday rotator, a first quarter-wave plate and a second quarter-wave plate, a polarization splitter, a light source, and a signal processing circuit including a photoelectric conversion element (13a, 13b); and the light incident and emitting unit is constituted by two waveguides arrayed; the light incident and emitting unit, the polarization splitter, the first quarter-wave plate, the second quarter-wave plate, the Faraday rotator, and the optical fiber for a sensor are arranged in order; the optical fiber for a sensor is birefringent and, is provided around a periphery of an electric conductor through which measured electric current flows, and includes a first end into which two circularly polarized lights (LC1, LC2) having different rotation directions for being incident and a second end for reflecting the incident circularly polarized lights (LC1, LC2); the second quarter-wave plate is provided on the first end side of the optical fiber for a sensor; the Faraday rotator is arranged between the first end side of the optical fiber for a sensor and the second quarter-wave plate; a reciprocating optical path is set to reciprocate the light through the polarization splitter, the first quarter-wave plate, the second quarter-wave plate, the Faraday rotator, and the optical fiber for a sensor by reflection of the light emitted from the light source on the second end; two linearly polarized lights are propagated in the reciprocating optical path between the first quarter-wave plate and the second quarter-wave plate, and the reciprocating optical path is set to compensate a phase difference between the two linearly polarized lights in the reciprocating optical path between the first quarter-wave plate and the second quarter-wave plate; a Faraday rotation angle of the Faraday rotator whose thickness of Faraday elements is optimized, at time of magnetic saturation is set to 22.5° + α° at a temperature of 23°C., such that the fluctuation range of a ratio error in a measurement value of the measured electric current output from the signal processing circuit is set within a range of ±0.5%; directions of crystal axes (X41, X51) on respective optical surfaces of the first quarter-wave plate and the second quarter-wave plate are set to be perpendicular to each other or in an equal direction; and a direction of a crystal axis on an optical surface of the polarization splitter and the direction of the crystal axis on the optical surface of the first quarter-wave plate are set to be perpendicular to each other or in an equal direction. a light incident and emitting unit, a optical fiber for a sensor, a Faraday rotator, a first quarter-wave plate and a second quarter-wave plate, a polarization splitter, a light source, and a signal processing circuit including a photoelectric conversion element (13a, 13b); and the light incident and emitting unit is constituted by two waveguides arrayed; the light incident and emitting unit, the polarization splitter, the first quarter-wave plate, the second quarter-wave plate, the Faraday rotator, and the optical fiber for a sensor are arranged in order; the optical fiber for a sensor is birefringent and, is provided around a periphery of an electric conductor through which measured electric current flows, and includes a first end into which two circularly polarized lights (LC1, LC2) having different rotation directions for being incident and a second end for reflecting the incident circularly polarized lights (LC1, LC2); the second quarter-wave plate is provided on the first end side of the optical fiber for a sensor; the Faraday rotator is arranged between the first end side of the optical fiber for a sensor and the second quarter-wave plate; a reciprocating optical path is set to reciprocate the light through the polarization splitter, the first quarter-wave plate, the second quarter-wave plate, the Faraday rotator, and the optical fiber for a sensor by reflection of the light emitted from the light source on the second end; two linearly polarized lights are propagated in the reciprocating optical path between the first quarter-wave plate and the second quarter-wave plate, and the reciprocating optical path is set to compensate a phase difference between the two linearly polarized lights in the reciprocating optical path between the first quarter-wave plate and the second quarter-wave plate; a Faraday rotation angle of the Faraday rotator whose thickness of Faraday elements is optimized, at time of magnetic saturation is set to 22.5° + α° at a temperature of 23°C., such that the fluctuation range of a ratio error in a measurement value of the measured electric current output from the signal processing circuit is set within a range of ±0.5%; directions of crystal axes (X41, X51) on respective optical surfaces of the first quarter-wave plate and the second quarter-wave plate are set to be perpendicular to each other or in an equal direction; and a direction of a crystal axis on an optical surface of the polarization splitter and the direction of the crystal axis on the optical surface of the first quarter-wave plate are set to be perpendicular to each other or in an equal direction.