A method for suturing a gastric wall including: inserting a flexible elongated loop-forming member perorally into a stomach advancing the loop-forming member along a lesser curvature line of the stomach to contact with the gastric wall in vicinity of the pyloric antrum, such that the loop-forming member returns along a greater curvature line of the stomach to a predetermined position and connects to a distal end portion of an overtube to form a closed loop in the stomach suctioning air from the stomach after forming the closed loop such that an anterior gastric wall is in proximity with a posterior gastric wall within a region encircled by the closed loop and suturing at least a portion of the anterior gastric wall to the posterior gastric wall that are in proximity in the region encircled by the closed loop.
