A control device for motor generator is provided in which different field current limit values are set in a low rotation speed zone and a high rotation speed zone in power generation, thereby enabling improvement in the degree of freedom in design, and in which continuous operation can be safely carried out since Ifg in power generation at high temperature is made smaller than Ifm in electric driving. In the control device for motor generator in which an engine is started and power generation is performed while a vehicle is running, a field current limit value Ifm in electric driving to start the engine is larger than a field current limit value Ifg in power generation. In the power generation, an inverter mode in a low rotation speed zone for boosting and an alternator mode in a high rotation speed zone for rectifying and outputting a generated voltage without boosting are provided. A field current limit value Ifgi in the inverter-mode power generation and a field current limit value Ifga in the alternator-mode power generation are set differently from each other, and the larger value is set as the value Ifg.