To eliminate framework from viewpoints of reduction of labor and risk of work as well as saving resources in the case a drain gutter is constructed on the housetop of a building or in floor, etc., of the basement and to provide a jig capable of easily constructing the beautiful finished drain gutter and a construction method of the drain gutter making use of the jig.

A drain gutter sheathing installation jig is equipped with an F joiner 11 as a floor fixing member fixed to a floor, a U joiner 12 as a sheathing holding member for holding a sheathing and a bolt 3 and a nut 12a as a slide mechanism for the slide holding member to vertically slide to the floor fixing member. A drain gutter construction method is constituted of process for fixing a leveler 10 to a required position on a floor Y by using the leveler 10 as the drain gutter sheathing installation jig, process for adjusting and fixing the leveler 10 to a specific level, process for setting the sheathing 20 (extrusion cement molding plates 20b and 20c) to the leveler adjusted and fixed to the specific level and process of works for casting cinder concrete C up to the specific height to the outside of the sheathing 20 to be set.