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, the series mode is suspended to temporarily stop fuel supply to the engine, and the motor generator forcedly drives the engine using electric power supplied from a driving battery to perform motoring, and failure determination of the O2 sensors is performed based on oxygen concentrations of an exhaust gas from the engine detected by a front O2 sensor and a rear O2 sensor when the fuel supply to the engine is stopped and when the fuel supply is restarted after finish of the motoring.