The causes of deaths in traffic accidents are predominantly human factors such as careless or "heedlessness" driving; recently, accidents that are believed to be due to deteriorated physical conditions, such as heart attacks, have been reported. Non-contact electrocardiography (ECG) monitor for continuous ECG detection while driving is needed to reduce a number of fatal accident by human error like this. Recently there are a lot of papers to detect cardiac electricity using capacitance coupling between human body and electrode. This sensor system must be adopted appropriate high input impedance circuit and noise reduction technique as a function of source impedance value especially for a seat mounted sensor.We investigated the impedances of the intermediary materials such as a seat cover and clothing for installing vehicle use by dielectrics measurement system can measure extra-high impedance (100TΩ), and designed an effective circuit configuration for common noise resistance, considering the effect of the sitting posture according to their results. The sensor system employed the DRL (Driven Right Leg) circuit is composed of two sensor head mounted into the seat cover and a electrode mounted on a steering wheel for reducing well common mode noise in cardiac electricity frequency.We have succeeded in attaining stable detection of cardiac electrical impulses during two driving conditions, stable driving at the average speed of 70km/h and driving with frequent changes at the speed of 0-30km/h, operations of the accelerator, brake, steering wheel, and direction indicator.