The obstructions such as piles and rocks ahead of a shield machine must be detected quickly and dealt with because they disturb the digging work remarkably. An underground radar system installed on the front of a shield machine is used to solve the problems. The radar radiates electromagnetic waves ahead of the shield machine and checks the reflected waves. However it is difficult to detect an obstruction in the distance because electromagnetic waves attenuate rapidly in the earth. To compensate the disadvantage, we multiplied the antennas of the radar, confirmed the performance in ground experiments, and strengthened the electromagnetic wave. We also checked and improved the function. As a result, the exploratory signal has been increased by 17 dB, enabling the detection at a distance.