To improve welding efficiency by sequentially welding a plurality of materials to be welded in vacuum where high vacuum condition is maintained.

A resistance welding device in vacuum includes electrode units 15 capable of mounting the materials W to be welded and having lower electrode chips 15 A that are mutually independently vertically moved. The plurality of the materials to be welded are preliminarily mounted at the electrode unit outside a vacuum chamber 5, the electrode unit is mounted at a support member 19 in the vacuum chamber, and the plurality of the materials to be welded are collectively transferred into the vacuum chamber. When the inside of the vacuum chamber reaches a predetermined vacuum condition, the electrode unit is intermittently moved, the materials to be welded are sequentially transferred between an upper welding electrode and a lower welding electrode and resistance welding is carried out to perform vacuum airtight sealing. After the plurality of the materials to be welded in the vacuum chamber are resistance welded, airtight sealing devices that are airtight sealed in vacuum are collectively carried out from the vacuum chamber by the electrode unit.