A Rankine cycle system with an abnormality detection apparatus that includes: a main body of an internal combustion engine which is configured to vaporize 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, is configured to separate the cooling medium into a gas-phase cooling medium and a liquid-phase cooling medium; a superheater that, downstream of the gas-liquid separator, is configured to give waste heat of the internal combustion engine to the cooling medium; a waste heat recoverer that, downstream of the superheater, is configured to recover energy of the waste heat that the cooling medium has obtained; a condenser that, downstream of the waste heat recoverer, is configured to condense the gas-phase cooling medium; a header tank that is disposed downstream of the condenser, and that stores the liquid-phase cooling medium; and a pumper that is positioned between the header tank and the main body and that is configured to pump the liquid-phase cooling medium in the header tank to the main body, the abnormality detection apparatus characterized by comprising: a cooling medium amount detector that is configured to detect amount of the liquid-phase cooling medium in the gas-liquid separator; a temperature detector that is configured to detect temperature of the gas-phase cooling medium in the superheater; a header tank cooling medium amount detector that is configured to detect an amount of the liquid-phase cooling medium in the header tank; a first abnormality detection portion that is configured to detect abnormality of a thermostat and a water pump that operate together to function as a cooling medium supply portion that supplies the cooling medium to the main body; and a second abnormality detection portion that is configured to detect abnormality of the pumper based on 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 header tank if abnormality is not detected by the first abnormality detection portion. a main body of an internal combustion engine which is configured to vaporize 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, is configured to separate the cooling medium into a gas-phase cooling medium and a liquid-phase cooling medium; a superheater that, downstream of the gas-liquid separator, is configured to give waste heat of the internal combustion engine to the cooling medium; a waste heat recoverer that, downstream of the superheater, is configured to recover energy of the waste heat that the cooling medium has obtained; a condenser that, downstream of the waste heat recoverer, is configured to condense the gas-phase cooling medium; a header tank that is disposed downstream of the condenser, and that stores the liquid-phase cooling medium; and a pumper that is positioned between the header tank and the main body and that is configured to pump the liquid-phase cooling medium in the header tank to the main body, a cooling medium amount detector that is configured to detect amount of the liquid-phase cooling medium in the gas-liquid separator; a temperature detector that is configured to detect temperature of the gas-phase cooling medium in the superheater; a header tank cooling medium amount detector that is configured to detect an amount of the liquid-phase cooling medium in the header tank; a first abnormality detection portion that is configured to detect abnormality of a thermostat and a water pump that operate together to function as a cooling medium supply portion that supplies the cooling medium to the main body; and a second abnormality detection portion that is configured to detect abnormality of the pumper based on 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 header tank if abnormality is not detected by the first abnormality detection portion.