An electric current measuring apparatus comprising: a signal processing circuit which includes at least an optical fiber for a sensor , a polarization separating unit , a Faraday rotator , a light source , and a photoelectric conversion element , wherein the optical fiber for a sensor is provided around the periphery of an electrical conductor through which an electric current (I) to be measured flows, and has one end at which linearly polarized light (LO) is incident and the other end at which the incident linearly polarized light (LO) is reflected, the polarization separating unit is provided at the one end of the optical fiber for a sensor , the Faraday rotator is disposed between the one end of the optical fiber for a sensor and the polarization separating unit , characterized in that the Faraday rotation angle of the Faraday rotator at the time of magnetic saturation is set to degrees of 22.5° + α° at a temperature of 23°C, a curve of temperature characteristics of a ratio error in a measurement value of an electric current (I) to be measured, output from the signal processing circuit is shifted to a high-temperature side by adjusting the Faraday rotation angle, such that the fluctuation range of the ratio error in the measurement value of the measured electric current (I) output from the signal processing circuit is set within a range of ± 0.5% , said fluctuation range being decreased compared to a corresponding fluctuation range at a Faraday rotation angle of 22.5°, and a temperature range, in which the fluctuation range is set within a range of ±0.5%, is between -20°C or higher and 80°C or lower. a signal processing circuit which includes at least an optical fiber for a sensor , a polarization separating unit , a Faraday rotator , a light source , and a photoelectric conversion element , wherein the optical fiber for a sensor is provided around the periphery of an electrical conductor through which an electric current (I) to be measured flows, and has one end at which linearly polarized light (LO) is incident and the other end at which the incident linearly polarized light (LO) is reflected, the polarization separating unit is provided at the one end of the optical fiber for a sensor , the Faraday rotator is disposed between the one end of the optical fiber for a sensor and the polarization separating unit , characterized in that the Faraday rotation angle of the Faraday rotator at the time of magnetic saturation is set to degrees of 22.5° + α° at a temperature of 23°C, a curve of temperature characteristics of a ratio error in a measurement value of an electric current (I) to be measured, output from the signal processing circuit is shifted to a high-temperature side by adjusting the Faraday rotation angle, such that the fluctuation range of the ratio error in the measurement value of the measured electric current (I) output from the signal processing circuit is set within a range of ± 0.5% , said fluctuation range being decreased compared to a corresponding fluctuation range at a Faraday rotation angle of 22.5°, and a temperature range, in which the fluctuation range is set within a range of ±0.5%, is between -20°C or higher and 80°C or lower. <EOS>