To provide an inner periphery welding device with which welding work between a steel pipe such as a long size bent pipe and branch pipe and a pipe flange is efficiently executed and the stabilization of quality is realized.

This device has a flange fixing block 102 and chain tensioner 104 for fixing the inner periphery welding device 100 to the flange 1, a torch frame 113 which is rotated along a virtual plane which intersects an axis of the steel pipe by the rotation of a driving motor 108 provided on a slide frame 106, torch mount 116 which is supported on this torch frame 113 movably along a direction (the direction of an arrow B) intersects the axis of the steel pipe 2 and an angle adjusting mechanism for holding a welding torch 22 and also adjusting an facing angle (the direction of an arrow D) of the welding torch 22 to a place to be welded and is provided with a holding plate 120 which is supported on the torch mount 116 so as to be vertically movable along the axial direction (the direction of an arrow C) of the steel pipe 2.