The present invention discloses a welding device which comprises: a first steel pipe provided to be welded and a second steel pipe provided to face the first steel pipe; a space maintaining unit which is provided between the first and second pipes to maintain a fixed space between the first and second steel pipes; a tact-welding unit that is located in between the first and second steel pipes and which tact-welds the first and second steel pipes; a torch unit which welds a connection part apart from the tact-weld region of the first and second steel pipes; and a steel pipe rotating unit which rotates the first and second steel pipes to use the torch unit for welding the connection part. The welding unit performs a welding work by maintaining a constant root gap between steel pipes where a welding work is performed, and improves the productivity of a welding work by improving welding efficiency. The welding efficiency is also improved by automatically or semi-automatically rotating the steel pipes.