The present invention simply and efficiently extracts and produces useful substances such as astaxanthin, silk proteins, and polyphenols from a plant material abundantly containing these useful substances. A material such as silk proteins or plant materials containing a useful substance is: immersed in an alkaline aqueous solution which is of a neutral salt and which is obtained by adding a proper amount of an alkaline agent to an aqueous solution of, for example, sodium chloride, calcium chloride, or the like； and reductively decomposed using the reduction potential generated on the alkaline side of the aqueous solution of a neutral salt to extract the useful substance in a state of being dissolved in the aqueous solution of a neutral salt.
