An abnormality detection apparatus for a Rankine cycle system, characterized by comprising: a main body of an internal combustion engine which vaporizes a cooling medium that circulates in the internal combustion engine by giving waste heat to the cooling medium; a gas-liquid separator that, downstream of the main body, separates the cooling medium into a gas-phase cooling medium and a liquid-phase cooling medium; a superheater that, downstream of the gas-liquid separator, gives waste heat of the internal combustion engine to the gas-phase cooling medium and the liquid-phase cooling medium; a waste heat recoverer that, downstream of the superheater_, recovers energy of the waste heat that the cooling medium receives; a control valve provided in a liquid-phase cooling medium passageway which connects the gas-liquid separator and the superheater_, and through which the liquid-phase cooling medium separated by the gas-liquid separator passes; a cooling medium amount detector that detects amount of the liquid-phase cooling medium in the gas-liquid separator; a temperature detector that detects temperature of the gas-phase cooling medium in the superheater; a cooling medium supply abnormality detection portion that detects whether there is abnormality of a cooling medium supply portion that supplies the cooling medium to the main body; and a control valve abnormality detection portion that detects whether there is abnormality in the control valve based on the amount of the liquid-phase cooling medium in the gas-liquid separator and the temperature of the gas-phase cooling medium in the superheater if abnormality in the cooling medium supply abnormality detection portion is not detected. a main body of an internal combustion engine which vaporizes a cooling medium that circulates in the internal combustion engine by giving waste heat to the cooling medium; a gas-liquid separator that, downstream of the main body, separates the cooling medium into a gas-phase cooling medium and a liquid-phase cooling medium; a superheater that, downstream of the gas-liquid separator, gives waste heat of the internal combustion engine to the gas-phase cooling medium and the liquid-phase cooling medium; a waste heat recoverer that, downstream of the superheater_, recovers energy of the waste heat that the cooling medium receives; a control valve provided in a liquid-phase cooling medium passageway which connects the gas-liquid separator and the superheater_, and through which the liquid-phase cooling medium separated by the gas-liquid separator passes; a cooling medium amount detector that detects amount of the liquid-phase cooling medium in the gas-liquid separator; a temperature detector that detects temperature of the gas-phase cooling medium in the superheater; a cooling medium supply abnormality detection portion that detects whether there is abnormality of a cooling medium supply portion that supplies the cooling medium to the main body; and a control valve abnormality detection portion that detects whether there is abnormality in the control valve based on the amount of the liquid-phase cooling medium in the gas-liquid separator and the temperature of the gas-phase cooling medium in the superheater if abnormality in the cooling medium supply abnormality detection portion is not detected.