PURPOSE: To increase the mechanical strength and raise the stability by putting box bodies into a frame of a united construction when a water gate to be used for adjustment of irrigation water in a river, for water diversion or the like is formed.

CONSTITUTION: A form frame 7 is fixed on a forming bed 6 and reinforced fibers, thermosetting resin and the like are filled up in required thickness around the frame. After setting, a box body 1 and a box body 2 are removed from a form. Then, the inside part of the frame 4 of the box body 1 is ground to remove the projected resin and make the part easy to bond, while the outside part of the frame 5 of the box body 2 is given a similar grinding treatment. The ground places of the box bodies 1 and 2 are coated with thermosetting resin or the like in the main and are matched and put together.