An abnormality detection apparatus for a Rankine cycle system includes: a gas-liquid separator that separates a cooling medium that circulates in an internal combustion engine into a gas-phase and a liquid-phase; a superheater that gives waste heat of the engine to the cooling medium; a waste heat recoverer that recovers energy of waste heat of the cooling medium; a valve provided in a passageway; a liquid surface sensor that detects liquid surface level of the liquid-phase cooling medium in the gas-liquid separator; a vapor temperature sensor that detects temperature of the gas-phase cooling medium in the superheater; and an ECU that determines whether the valve is abnormal on the basis of the amount of the liquid-phase cooling medium and the temperature of the gas-phase cooling medium if abnormality of the cooling medium supply portion of the engine is not detected.