In a nuclear fusion experiment device, a plasma discharge is to be sustained for a long time in steady state operation. In such a device an electromagnetic sensor that has a signal integrator to measure direct currents will cause a technical problem of zero point drift on signals. Therefore, the detection device using new technology for direct current measurement, optical current transformer (optical CT), was developed. The device has an optical fiber specified for850 and 1550nm (wavelength) were manufactured, and was applied to current measurement in JT-60U experiments. A gamma ray irradiation examination was also done to the optical CT.