Hybrid-vehicle drive system with torque increasing device and driving method thereof

In a hybrid vehicle drive system, an output shaft (2) of an internal combustion engine is connected to a first motor generator (MG1) and a wheel drive shaft through a power distribution mechanism, and the wheel drive shaft is connected to a second motor generator (MG2). The hybrid vehicle drive system is provided with a device that increases the torque of the wheel drive shaft separately from the second motor generator when the internal combustion engine is cranked by the first motor generator.