In a hybrid vehicle that selects a series mode in which an engine drives a motor generator to generate electric power and a driving motor drives drive wheels, fuel supply to the engine is stopped and motoring in which the motor generator forcedly drives the engine can be performed, failure determination of a front O2 sensor and a rear O2 sensor provided in an exhaust passage of the engine can be performed based on a change in a detection value of the sensors when the fuel supply is stopped, and a throttle valve of the engine is forcedly opened in the motoring.