An object of the present invention is to provide an industrially applicable method for producing acrolein via treatment with supercritical water from glycerin obtained as a by-product during the process of biodiesel fuel production from waste animal/plant fat or oil with the use of an alkali catalyst.The present invention provides a method for producing acrolein, which comprises: a determination step of determining the hydrogen ion concentration in glycerin obtained as a by-product; an acid addition step of adding to the glycerin an acid at an amount calculated based on the results of the determination step so as to make the glycerin acidic; and a supercritical water treatment step of allowing supercritical water to act on the glycerin after the acid addition so as to produce acrolein from the glycerin.