Recently, a magneto impedance effect found in amorphous wires with soft magnetic properties has been noted as a new principle in the sensing magnetic field. According to this effect, the impedance of the wire in the range of high frequencies over 10 MHz changes remarkably with an external magnetic field. This effect is expected to be promising for magnetic field sensors with high sensitivity. Therefore, we have attempted to introduce this effect into amorphous thin films to extend the application fields, and a novel thin film sensor sensitive to a small magnetic field based on the magneto impedance effect has been proposed. The sensor consists of two individual detecting elements with FeCoSiB/Cu/FeCoSiB multi-layers which forms a half bridge. The detecting element exhibits a large impedance change ratio of more than 100% when an external magnetic field is applied. By optimizing of the operating point via a bias field and processing the signal with a synchronous rectifier circuit, no hysteresis, good linearity and good stability even with temperature variation as well as high sensitivity in the sensor characteristics have been achieved. The variation in the sensor output with the temperature is largely reduced to one-third, compared to the conventional thin film sensor we developed previously. A detection resolution of 10^-3 Oe order higher than those of any other conventional thin film sensors is obtained.