PURPOSE: To enable easy detection of side swaying and its magnitude of an underground tunnel propulsor, by installing two detecting coils into a propulsor which is allowed to propel in the center right below guide lines and by detecting the side swaying and its magnitude of the propulsor by a difference of detected voltages of each detecting coil.

CONSTITUTION: Two parallel guide lines 3a, 3b are arranged on the ground surface which allow electric currents of different frequencies to flow for guiding an underground tunnel propulsor 1 and two detecting coils 4a, 4b are mounted onto the propulsor 1 for propelling in the center just below the guide lines, and side swaying and its magnitude of the propulsor are detected by a difference of detecting voltages of each coil. Namely, two guide lines 3a, 3b which permit themselves to be run-over by vehicles, etc. and admit flow of currents of different frequencies are arranged on the ground surface and the direction of side swaying of the underground tunnel propulsor 1 is detected by the guide lines their frequencies being different from those of the two detecting coils 4a, 4b.