Abstract: A process for producing large-sized square steel pipe includes the steps of continuously paying off steel plate from a roll of the same, removing strain from the plate while conveying the plate, and cutting the plate to predetermined lengths. The lengths of plate are successively bent into a shape resembling a rectangular or square pipe having corner portions with angles slightly larger the 90.degree.. The successive plates are tack-welded and opposed outer rollers are pressed against the outer sides of the plate while the plate is conveyed longitudinally thereof. The opposed edges of the plate are thereafter welded together by high frequency welding.