A control device of a premix combustion engine, the device comprising: a hybrid turbocharger comprising a turbocharger having a compressor and a turbine, and a motor generator coupled with the compressor via a rotating shaft; the premix combustion engine in which air is supplied from the compressor, and from which exhaust gas is discharged toward the turbine, a generator coupled with the premix combustion engine via a rotating shaft, and a control unit for controlling the motor generator based on operating conditions of the generator or the premix combustion engine, an exhaust gas bypass unit having a bypass passage and a flow rate regulating valve provided on the bypass passage so as to adjust the output of the turbine, a necessary airflow rate computing unit for calculating a necessary airflow rate required for the operation of the premix combustion engine, a charging air measuring unit for measuring the airflow rate of the air supplied to the premix combustion engine from the compressor, and an arithmetic unit for calculating a power output or a rotation speed of the hybrid turbocharger and performing a feedback control regarding the power output or the rotation speed of the hybrid turbocharger so that a measurement airflow rate measured by the charging air measuring unit is in consonance with a necessary airflow rate of the premix combustion engine calculated by the necessary airflow rate computing unit, wherein the control unit controls the power output or the rotation speed of the hybrid turbocharger based on the calculated results obtained by the arithmetic unit, and wherein the flow rate regulating valve is controlled so as to control the opening and closing of the bypass passage in response to the generating power or the rotation speed of the hybrid turbocharger, and the flow rate regulating valve is operated so that the bypass passage is opened when the generating power of the hybrid turbocharger exceeds a prescribed maximum level. a hybrid turbocharger comprising a turbocharger having a compressor and a turbine, and a motor generator coupled with the compressor via a rotating shaft; the premix combustion engine in which air is supplied from the compressor, and from which exhaust gas is discharged toward the turbine, a generator coupled with the premix combustion engine via a rotating shaft, and a control unit for controlling the motor generator based on operating conditions of the generator or the premix combustion engine, an exhaust gas bypass unit having a bypass passage and a flow rate regulating valve provided on the bypass passage so as to adjust the output of the turbine, a necessary airflow rate computing unit for calculating a necessary airflow rate required for the operation of the premix combustion engine, a charging air measuring unit for measuring the airflow rate of the air supplied to the premix combustion engine from the compressor, and an arithmetic unit for calculating a power output or a rotation speed of the hybrid turbocharger and performing a feedback control regarding the power output or the rotation speed of the hybrid turbocharger so that a measurement airflow rate measured by the charging air measuring unit is in consonance with a necessary airflow rate of the premix combustion engine calculated by the necessary airflow rate computing unit, wherein the control unit controls the power output or the rotation speed of the hybrid turbocharger based on the calculated results obtained by the arithmetic unit, and wherein the flow rate regulating valve is controlled so as to control the opening and closing of the bypass passage in response to the generating power or the rotation speed of the hybrid turbocharger, and the flow rate regulating valve is operated so that the bypass passage is opened when the generating power of the hybrid turbocharger exceeds a prescribed maximum level.