There is disclosed a method for the plasma arc welding of corrugate membranes, which can prevent a welding failure from occurring around the corrugate portion. In this welding method, NC data for robot control are modified so as to gradually change a torch angle around the R portions at a rate not greater than the maximum angular velocity and increase a welding speed around the R portions of foot regions of corrugate portions 1B and 2B and the R portion of an apex 22 before a welding torch 12 reaches the R portions. Three parameters are prepared for determining the deformation of the corrugate portions 1B and 2B in terms of height, width of the foot regions and the swell of the trunk portion. Data indicative of the height and foot width of the corrugate portion measured by a laser displacement sensor 15 are compared with standard corrugation NC data, and the NC data for robot control are modified to correct the height, foot width and swell degree thereof by utilizing the parameters.