To manufacture short welded steel pipes at high efficiency and high yield and at a low cost by shearing a steel sheet before roll forming, imparting steps with a prescribed interval and, after roll forming, reducing the side faces of a steel pipe obtained by welding the steel sheet and cutting off the pipe in the stepped part.

The steps are prepared by vertically shifting the steel sheet in the thickness direction with a rotary shear 3 having the step on the outer peripheral part and the steel sheets are connected in a degree at which the steel sheet are not broken at the time of roll forming. The interval between the steps is decided in accordance with the final pipe length by taking elongations at the time of roll forming and sizing into consideration. Next, the steel sheet is formed into a tubular form with a roll forming machine 4, welded with a welding device 5, pressurized with squeezing rolls and the bead formed is cut and removed with a cutting tool 6. Moreover, by cooling the steel pipe with a cooling system 7, improving dimensional accuracy with a sizing machine 8, pressing the pipe in the vicinity of a cutting plane with a supporting device 9 and cutting the steel pipe at the stepped part by pressing a cutting press 10 against from the side face of the steel pipe, the welded steel pipe 12 is obtained.