The present invention relates to a method for reducing the relative ratio of 3-hydroxy-3,4-didehydro-β,ψ-caroten-4-one (HDCO) to astaxanthin in a composition containing astaxanthin and HDCO by contacting the composition with an acidic medium having a pH of 3 or less and/or a basic medium having a pH of 9 or greater, and also relates to a method for producing an astaxanthin-containing composition which includes reducing the relative ratio of HDCO by the above method. By means of the method of the present invention, the relative ratio of HDCO, the biological function of which is not known, in an astaxanthin-containing composition can be easily reduced.
