A means for heat bonding a large number of plastic branch pipes to a single plastic main pipe in such a manner that the branch pipes are perpendicular to the main pipe is configured so that the bonded portions are uniform with respect to each other, and each working process is automated mechanically. A branch pipe-side fixed table (1T) is provided on the front side of a base (1) and a main pipe-side fixed table (1T') is provided on the rear side of the base (1) in such a manner that there is a gap (S) between both the fixed tables (1T, 1T'). A movable table (2) for branch pipes is provided on the branch pipe-side fixed table (1T) so as to be slidable forward and backward, and a movable table (3) for a main pipe is provided on the main pipe-side fixed table (1T') so as to be slidable forward and backward. A branch pipe clamp device (5) is provided on the movable table (2), and a main pipe clamp device (6) is provided on the movable table (3). A setting plate device (5R) ascending from below and a heating device (9) descending from above are arranged in the gap (S). The movable table (2) for branch pipes and the movable table (3) for a main pipe are moved forward and backward, and by utilizing this movement, a group of branch pipes (8B) and mounting holes (8G) in a main pipe (8A) are melted by both surfaces of the heating device (9) having descended. Then, the heating device (9) is lifted and withdrawn, and the edges (eb) of the branch pipes (8B) and the mounting holes (8G) in the main pipe (8A) are pressed and heat bonded together.