An economical method of manufacturing an elongated composite diffraction grating in mass production with the aid of a combination of a plurality of steps comprises replicating from a master diffraction grating a number of replica diffraction gratings, clamping the replica diffraction gratings in a jig with one of them being made stationary while the other is made adjustable so as to bring it into coincidence with the stationary one in direction and phase, and mechanically securing or cementing the gratings together.