In recent years, the fuel consumption improvement of automobiles is indispensable due to the global move to reduce CO₂ emissions. Downsizing of engines by turbocharger obtains the output equal to a large engine, and improves the fuel consumption. However, turbocharger has the response delay called turbo lag. In order to improve a transient response, we developed a high-speed motor. The electric supercharger consists of high-speed motor and compressor, and drives compressor by high-speed motor instead of an exhaust gas turbine. By combining conventional turbocharger and electric supercharger, we developed two-stage turbocharger system.In this paper, we explained development of high-speed motor and inverter. And, as application example, we explained electric two-stage turbocharger system. The high-speed motor and inverter are connected without harness, and assembled in one single unit. The motor is driven at high efficiency by vector control. The performance test shows a high speed response of 0.7 sec acceleration time from zero to maximum speed 90,000 rpm. Electric two-stage turbocharger, applying high-speed motor, is evaluated by engine simulation tool (GT-Power) and engine bench test. As a result of engine test using a 1.5-liter gasoline engine, the electric two-stage turbocharger demonstrated the 43% improvement in a response time at 1500 rpm, compared with normal two-stage turbocharger. This experiment shows that electric supercharger is effective to improve transient response. It is expected to contribute ever tightening CO₂ reduction with automobile engines from now on.