To improve production efficiency by arranging a rotary cutter and an inside surface bead cutter in series on the steel pipe discharging side of an electric resistance welded tube manufacturing device, and arranging them so as to respectively and independently operate.

A rotary cutter A is accelerated, and is made to travel in the leftward direction, and when it reaches a prescribed position of a cutting length, it becomes the same speed as a production speed of a steel pipe 1. The steel pipe 1 is cut by actuating a rotary disk cutter 9 by driving a driving motor M2. An inside surface bead cutter B is accelerated, and is made to travel from a position stopped in the origin by driving a traveling motor M4 by a command signal from a length measuring device, and when it reaches a cutting position, it becomes the same speed as the production speed. A welding bead connected on the inside of the steel pipe is cut by a cutting edge by extendedly driving a cylinder of a bead cutter D.