Herein is disclosed a method for producing mixed crystals of disodium 5′-guanylate and disodium 5′-inosinate which comprises feeding a mixed solution of disodium 5′-guanylate and disodium 5′-inosinate which solution will become supersaturated at the below-mentioned constant temperature, to a solution or slurry of disodium 5′-guanylate and disodium 5′-inosinate charged in a crystallization bath (lower-temperature bath) and kept at a constant temperature, whereby mixed crystals of disodium 5′-guanylate and disodium 5′-inosinate are deposited from the mixed solution of disodium 5′-guanylate and disodium 5′-inosinate, according to which method 5′-GMP2Na which is difficult to handle due to the properties and powder characteristic of its crystals in particular and 5′-IMP2Na, in the form of crystals which are easy to handle, that is, I+G mixed crystals having a given I/G ratio, can be produced under simple process control and with inexpensive facilities, with the I/G ratio being controlled easily.