An abnormality detection apparatus for a Rankine cycle system includes: a main body that vaporizes a cooling medium that circulates in an internal combustion engine by giving waste heat to the cooling medium; a gas-liquid separator that separates the cooling medium into a gas-phase and a liquid-phase; a superheater that gives waste heat of the engine to the cooling medium; a condensate header tank that stores the liquid-phase cooling medium; a vane pump that pumps the liquid-phase cooling medium. If abnormality of a cooling medium supply portion that supplies the cooling medium to the engine is not detected, the abnormality detection apparatus determines whether a vane pump has abnormality on the basis of the amount of the liquid-phase cooling medium in the gas-liquid separator, the temperature of the gas-phase cooling medium in the superheater and the amount of the liquid-phase cooling medium in the condensate header tank.