PURPOSE: To efficiently transmit a signal without being affected by the rotation of a boring head, noise at pressing of a press fit rod, the joint of rods and sea water and rock around the rod by adopting a signal transmission system due to electromagnetic induction.

CONSTITUTION: For example, a temperature sensor 1 is inbedded near the tip of a steel-made boring rod 7, a DC voltage proportional to the sensing temperature in the earth is outputted, the output is converted into a signal having a frequency in response to the input voltage by a voltage-frequency conversion circuit 2, modulates a carrier given from an oscillation circuit 3 by a modulation circuit 4, amplified by an amplifier circuit 5 and fed to a transmission coil 6. The transmission coil 6 is provided near the tip of the boring rod 7 and a magnetic flux change is given by a drive signal flowing through the coil. The detection coil 8 is provided to the other end of the boring rod 7 and an induced voltage from the coil 8 is detected by a carrier band, the detected signal is amplified by an amplifier circuit 9 and demodulated by a demodulation circuit 10 as the modulation signal. Thus, the long range underground transmission with excellent S/N is attained.