[Problems to be solved by the invention]A method and sensor for detecting the fluorides, chlorides or bromides of hydrocarbons such as C5F8 or C4F6 are provided, which are capable of the detection with quickness and high sensitivity at a room temperature or a temperature closer thereto, not a high temperature, without interference with hindrance gases from fluorine liquids or the like.[Means for solving the problems]A detection agent, detection method, and detection sensor based on an optical change or mass change by a direct reaction with a nitrogen compound having at least two rings at which an amidine backbone is centered, which is represented by the following general formula (I).