[en] A water-cooled high-power rotating disk target is employed at the in-flight radioactive-isotope (RI) beam separator (BigRIPS), RIKEN. For a ²³⁸U primary beam with energy of 350 A MeV and an intensity of 1 particle μA bombarding a beryllium or carbon target disk with a 1 g/cm² thickness, a total heat loss of 28 GW/m² on the target surface is estimated for a beam-spot size of 1 mm in diameter. For the development of a suitable target assembly, a test bench was constructed and has been used for the present RI beam separator (RIPS). A beam spot temperature monitor using an infrared thermo-viewer with telescope lens was developed. The measured temperature for some high-power beam experiments corresponded to results of computer simulations. A radiation hardness test of the magnetic fluid used for a vacuum-tight rotating actuator was performed by irradiating with intense γ-ray converted from an electron beam. No significant change of viscosity was observed up to the total dose of 0.7-1.8 MGy. Based on these studies, a practical target system for the BigRIPS separator was designed. To facilitate the maintenance of the target system in a high-radiation environment, a remote-handling maintenance cart system is considered