Disclosed is a method of manufacturing a substrate for an organic EL device, the method comprising the step of: filling grooves of the optical element with sol-gel coating solution or organic metal cracking solution when a diffraction grating 12 is formed on the glass substrate 11, wherein an encapsulation member 5 is mounted to the glass substrate 11 in order to fill the groove 12a with the coating solution, and the coating solution is injected into a gap between the encapsulation member 5 and the diffraction grating 12, so that the organic EL device can be stably manufactured with low variation between optical properties according to positions of the substrate and with improved luminous efficiency. Further, the present invention provides a method of manufacturing a substrate for an organic EL device having a glass substrate 11 with an diffraction grating 12, wherein the dispersion solution of particles is applied to the glass substrate 11 to form the diffraction grating 12, so that the organic EL device can be stably manufactured with low variation between the optical properties according to the positions of the substrate and with the improved luminous efficiency.