A breeder blanket, whose purposes are to breed tritium and to produce thermal energy, is indispensable for a fusion reactor. In addition to demonstrating the physics of burning D-T plasmas, an experimental reactor such as the ITER will also conduct a verification test of a blanket design. An in-pile functional test has been considered to obtain above engineering data since 1992 in the JMTR project. The sweep gas system which is a system to measure and recover the tritium released from a blanket mock-up has been designed. From the results of the design, the sweep gas system will be sufficient for the in-pile functional test in the JMTR.