A method of producing an unsaturated acid from a salt of a hydroxycarboxylic acid, the method includes providing an aqueous solution including a salt of a hydroxycarboxylic acid; adding a solvent with a boiling point of 200° C. or higher to the aqueous solution; and heating the aqueous solution at a temperature lower than the boiling point of the solvent to split a salt from the salt of the hydroxycarboxylic acid while simultaneously dehydrating the hydroxycarboxylic acid, thereby converting the hydroxycarboxylic acid to the unsaturated acid.