PURPOSE: To set a water seal pushing sheet to a joint body with simple operation by a method wherein in manufacture of a synthetic resin eaves-gutter joint, elastic deformation is given to the joint body, a water seal pushing sheet is made to run along the inner circumference of the joint body and is fused with the joint body at both its side ends.

CONSTITUTION: A joint body 1 is formed from hard synthetic resin in the form of a nearly semicircular section and fitting lugs 3 are set near the upper ends of both its side. A pushing sheet 2 is made from soft synthetic resin. In the manufacture, the joint body 1 is set in a form 4 giving bending deformation to the joint body 1 in order to reduce its inner diameter and the pushing sheet 2 is placed so that it can stick to the inner circumference of the joint body 1. Then, the joint body 1 is fused with the pushing sheet 2 at the arrow portions by means of high frequency fusion or ultrasonic fusion. Finally, the joint body 1 is taken out of the form 4 and is restored elastically to the original form. At this time, a gap 5 appears between the pushing sheet 2 and the joint body 1. Since the eaves-gutter joint manufactured in this way sticks to the pushing sheet 2, water is prevented from leakage.