PURPOSE: To measure horizontal displacement without scanning receiving coils by arranging two receiving coils symmetrically about a designed line and making the distance between the receiving coils and a pit approximately coincide with the distance between a magnetic field generating element fitted to an excavator and the pit.

CONSTITUTION: The magnetic field generating element 2 fitted to the inside of the tunnel excavator 1 is attached so that its center magnetic flux is generated always in the perpendicular direction. A pair of receiving coils 16', 16' are arranged on a ground position where the distance from the pit is approximately equal to the distance between the pit and the magnetic field generating element 2 symmetically about the designed line 7. Electric power induced by the magnetic field generated from the magnetic field generating element 2 is measured by a signal measuring device 10 such as a selecting level meter and inputted to an electronic computer, which outputs the horizontal displacement of the tunnel excavator from the designed line.