The highly sensitive field sensor with a Pockels crystal which was devised by us has been modified by using a light reflection technique. By replacing a metallic mirror with a dielectric mirror which is sputtered on one end face of the Pockels crystal, the new field measuring system has an advantage of precisely measuring the field distribution in the long gap discharge. This is the case when the field sensing part consisting of the Pockels crystal is isolated from the main field measuring system and both are connected to each other with a long light guide cable. Based on our experiments, we found that the measurable minimum field strength in the long gap discharge is 2 V/cm with the accuracy of 16 dB and 0.2 V/cm with the critical accuracy of 0 dB. Furthermore, we point out that the size of the field sensing part can be reduced down to 1 mm in diameter by using a dielectric mirror and a light guide cable.