To make bead cutting unmanned and improve productivity by providing a slide plate which moves up and down 1st and 2nd moving blades paired with 1st and 2nd fixed blades arranged right below and right above a line, and forming a through hole for the welded steel tube above the 2nd moving blades.

After a precedent steel tube 1a is cut by a cutoff device, the precedent steel tube 1a is pressed with a pinch roller 11b through the operation of a cylinder device 11c and conveyed at a speed higher than a line speed, so that it is parted from a following steel tube 1b. Once tube end detector 12a and 12b detect their interval being long enough to cut an internal surface cut bead 3, the slide plate moves down to cut the internal cut bead 3 with the 1st fixed blade 12e and 2nd moving blade 12g. The following steel tube 1b is conveyed through the through hole of the slide plate at a speed higher than the line speed and when the precedent steel tube 1a and following steel tube 1b are separated at a specific interval, the slide plate moves up to cut an internal cut bead with the 2nd fixed blade 12f and 2nd moving blade 12h.