In the Advanced Fusion Neutron Source (A-FNS), an accelerator-driven fusion-relevant neutron source that is planned for development in Japan, a few grams (3.5 g at full power operation) of tritium will be generated every year, mainly in the lithium target system. Since the generated tritium would migrate out of the lithium target system, it is necessary to estimate the tritium migration into and out of the lithium target system for the design of detritiation systems for the A-FNS. Therefore, a preliminary estimation is performed in the present study. As a result, it is found that almost all of the generated tritium in the lithium target system would be trapped in the impurity removal system, while less than 0.5% would migrate out. It is also indicated that the amount of tritium that would migrate out of the lithium target system would be able to be processed with the existing techniques so far.