A movable vacuum welding device (1) includes a vacuum chamber (7) having an edge section (12) opposite to a surface of a welding target (T) and forming a vacuum space between the surface of the welding target (T) and the vacuum chamber (7), a seal section (8) interposed between the edge section (12) and the welding target (T) throughout the entire circumference of the edge section (12), a welding head (9) configured to perform welding on the surface of the welding target (T) in the vacuum space, and a preload unit (10) configured to previously apply a load to the seal section (8). The seal section (8) includes a first seal member (27) formed of an elastic material and extending along the edge section (12), and a second seal member (28) disposed at least a rear side in a relative moving direction in which a welding bead (W) passes and having higher flexibility than the first seal member (27).