Abstract: A turbo charger system (32) for a diesel engine (1) with a simple structure, but improved in output, torque and fuel consumption rate. A high-stage turbine (3) and low-stage turbine (4) are provided in series on an exhaust gas line (2) of the engine (1). A high-stage compressor (6), which is connected to the high-stage turbine (3), and a low-stage compressor (7), which is connected to the low-stage turbine (4), are provided in series on an intake air line (5) of the engine (1). A bypass line (20) with a valve (21) is provided over the high-stage turbine (3). The bypass valve (21) is completely or substantially completely closed until the engine (1) reaches its maximum torque point (14) so as to raise a supercharging pressure. From the maximum torque point (14) to the maximum output point (15), the bypass valve (21) is gradually opened to adjust the supercharging pressure. The bypass valve is completely or substantially completely opened at the maximum output point (15) to suppress the supercharging pressure.