Using conventional synthesis methods, when an unsaturated carboxylic acid or a derivative thereof is synthesized from a hydroxycarboxylic acid or a derivative thereof by a dehydration reaction using a catalyst, the yield has been low. Accordingly, an object of the present invention is to provide a catalyst capable of synthesizing unsaturated carboxylic acids or derivatives thereof by a dehydration reaction that produces a high yield. The synthetic catalyst comprises an apatite compound containing an alkali metal in a crystal structure thereof, and the invention further includes methods for synthesizing an unsaturated carboxylic acid and/or a derivative thereof. The methods comprise contacting the aforementioned synthetic catalyst with the hydroxycarboxylic acid and/or a derivative thereof, so as to synthesize the unsaturated carboxylic acid and/or a derivative thereof by a dehydration reaction.