PURPOSE:To detect a propulsion tube with high accuracy by moving a transmitter on the ground surface nearly at right angles to the axial direction of the propulsion tube, sending a radio wave to right below and receiving the radio wave signal by a receiver, and measuring the arrival time and level of this radio wave signal. CONSTITUTION:The radio wave signal is sent to right below from the transmitter 3 fitted to a carriage 14 installed on the ground. The receiver 2 is installed on the top of the propulsion tube 1 and the radio wave signal received by the receiver 2 is amplified by an amplifier 12, sent to a signal processor 11 on the ground through a cable run through the propulsion tube 1 and a shaft 7, and displayed on a display device 6. Here, the transmitter 3 sends the impulsive radio wave intermittently while moving nearly at right angles to the axis of the propulsion tube 1. At this time, the value of the movement distance is obtained from the number of turns of the tires of the carriage 14 and sent to the processor 11. The process of transmission and reception is repeated to obtain a measurement result. Thus, the propulsion tube is detected with high accuracy.