An object of the present invention is to provide a method for commercially manufacturing acrolein in a large flow rate by making supercritical water and an acid interact with glycerin, wherein by efficiently mixing high-concentration glycerin and supercritical water with each other, the method is made capable of making the synthesis stably proceed with a high yield while the occlusion and abrasion of the pipes and devices due to the generation of by-products are being suppressed. The method for synthesizing acrolein of the present invention is a method for synthesizing acrolein by making supercritical water and an acid interact with glycerin, the method using a reaction apparatus including: a cylindrical mixing flow path for mixing a fluid including glycerin and a fluid including supercritical water with each other; a first inlet flow path, disposed offset from the central axis of the mixing flow path, for making the fluid including glycerin flow into the mixing flow path; and a second inlet flow path, disposed offset from the central axis of the mixing flow path, for making the fluid including supercritical water flow into the mixing flow path, wherein the first inlet flow path and the second inlet flow path are each provided in a plurality of numbers in such a way that the first inlet flow paths and the second inlet flow paths are alternately arranged so as to encircle the central axis of the mixing flow path.