In a propulsion system for hybrid vehicles, etc., a failure detection system that detects the occurrence of failure that obstructs a phase current supplied to an electric motor for propulsion. The direct current of the battery is converted to three-phase alternating current by an inverter and supplied to the electric motor. The controller drive controls the inverter so that the output torque of the electric motor attains a target torque. Each of the phase currents supplied to the electric motor are detected by a current sensor, and provided to a comparator. The comparator compares the value of each of the detected phase currents and the current limit value calculated by the current limit value calculation circuit, and outputs a signal showing that a current phase has abnormally risen. This signal is counted by the counter, and the occurrence of failure is identified based on this count value.