To provide a three-dimensional pipe assembly device capable of correctly welding a flange orthogonal to the axis of a diagonal pipe, and easily performing the welding assembly work of high accuracy in a short time by one worker without being affected by the skill of the worker.

The three-dimensional pipe assembly device for fitting a flange to an end of a bent pipe or a pipe with a branched pipe comprises a spindle 4 arranged horizontally at the predetermined height, a pair of main plates 5a, 5b having protractor shapes and provided on the spindle 4 movably in the axial direction, angle adjusting arms 10a, 10b adjacent to the main plates 5a, 5b, rockingly and pivotably supported by the spindle 4, and provided in a length adjustable manner, and flange fixing plates 23a, 23b provided on fore ends of the angle adjusting arms 10a, 10b to fix flanges 30a, 30b of the pipe.