Many studies on Electric Vehicle (EV), recently, are actively proceeded as one of the solutions of environmental problem. We are aiming at constructing vector control system for an Interior Permanent Magnet Synchronous Motor (IPMSM) that is employed a small EV for our test bed. In the vector control, since fundamental equations of IPMSM in stationary coordinate are transformed into rotational coordinate, IPMSM that is one of the AC motor is considered DC motor and various calculations are simplified. In order to apply the vector control, high resolution rotor position sensor is indispensable to detect precise information on rotor position and speed. However, position sensor of our IPMSM can detect 60[deg] at a minimum. Therefore in this study we examined a driving method of IPMSM by using information from low resolution position sensor combining with speed observer. Moreover, we have analyzed control system considering output voltage error that is a property of inverter. As a result our new method has improved speed response of the vector control system of the IPMSM.