Recent welding tests using either neutron irradiated or tritium charged material have shown that there was a good chance of success when there was no repeated heat cycle during welding, e.g., stringer bead welding, single pass laser welding, etc. However, the replacement/repair of the actual load carrying components often requires thick plate welding which can only be realized with multi-layer welding. In the present study, 20-mm-thick grooved plate of type 304 stainless steel was irradiated and then butt-welded by multi-layer welding. The weld heat input used was 1.0 and 2.0 MJ/m, which required 28 and 14 passes to fill up the groove. The samples were irradiated to 2.5×1021 ∼ 1.8×1023 n/m2 (E>1MeV), which resulted in 0.1∼1.6 appm of helium being produced. A conventional gas tungsten arc welding procedure was employed. The welded joints were subjected to the tensile test, side bend test, root bend test and cross-sectional metallography. The samples containing as much as 0.14 appm of helium passed all the tests; the mechanical properties fulfilled the standard requirement for the unirradiated base metal. At 0.6 appmHe and 1.0 MJ/m heat input, the welded joint showed sufficient strength, however, tiny intergranular cracking was observed in the HAZ where repetitive thermal cycles operated.