To obtain engineering data for a design of a fusion blanket, a program in which a mock-up simulating a blanket structure is irradiated is proceeded in the JMTR (Japan Materials Tasting Reactor) project.1 It is indispensable for this irradiation test to purge the tritium produced in the mock-up by the sweep gas and to measure the tritium concentration in the sweep gas on line. However, the bad effects such as tritium adsorption on the inner surface of the pipe and tritium diffusion from the pipe are apprehended, because the distance between the mock-up and an ionization chamber is about 20m long. Therefore, it is necessary to develop a sweep gas sensor which can measure the tritium concentration in the mock-up directly without the above apprehension. A sweep gas sensor had been fabricated on trial and several performance tests were carried out. From the results of the tests, it was found that this sensor could be used at the atmosphere of high temperature of 600–700°C and had good response.