A hybrid drive system of a vehicle, comprising a transmission (2) for transmitting the rotation of an input shaft from an output shaft to wheels after varying a speed, a first clutch (3) for disengaging and engaging the transmission of a power between the output shaft of an engine (1) and the input shaft of the transmission (2), a motor generator (4) commonly used as an electric motor and a generator, a second clutch (30) for disengaging and engaging the transmission of a power between the motor generator (4) and the input shaft of the transmission (2), a storage means (9) for storing an electric power fed from the motor generator (4), and a second clutch control device controlling the rotational speed of the motor generator (4) when the second clutch (30) is engaged and, after a difference in rotational speed between the motor generator and the transmission comes into an allowable range, engaging the second clutch (30), wherein when the motor generator (4) is stopped, the motor generator (4) is disengaged from a drive system by the second clutch to reduce a drive loss by the inertial mass and friction thereof, whereby the fuel consumption of the engine (1) can be improved.