An axial flow electrolytic cell includes an outer layer made of a polyolefin material and formed as a tube. A portion of the off-gas stream containing hydrogen sulfide is injected at the front end of the thermal reactor and in at least one other location downstream of the thermal reactor. When the to-be-rolled pipe is thread-rolled to a predetermined length, the oblique surface of the lever 44 moving in association with the movement of the abutment member 41 is gradually moved away from the cam member 45. Each synchronization device, for the main transmission, has two shift positions for respective gear ratio steps and a neutral position and the range group synchronization device has only two shift positions to shift between two gear ratios steps.