PURPOSE: To reduce the work load for measuring the horizontal displacement of a tunnel excavating machine and to improve working efficiency, by substituting the measurement of the horizontal position of the tunnel excavating machine with respect to a planned line under the ground into the measurement of horizontal displacement, and performing the measurement by an apparatus on the ground.

CONSTITUTION: A magnetic-field generating element 2 is attached in a tunnel excavating machine 1. An input signal having suitable amplitude and period is supplied to the element 2 by operating a signal supply device 3 through a cable 4. A radial alternate magnetic field 5, whose central magnetic flux is vertical, is generated. A magnetic-field detecting device 8 is provided at the aproximate position at an excavating distance on the planned line 7 so that a detecting guide 9 is perpendicular to the planned line 7. On the detecting guide 9, a magnetic- field detecting element 12, whose intercrossing plane 13 is vertical and which is supported by the detecting guide 9 at a right angle and can be linearly moved along the detecting guide 9, is attached. The magnetic-field detecting element 12 can detect the direction of arrival of the magnetic lines of force 5. The magnetic-field detecting element 12 is moved along the detecting guide 9, and the distance between the magnetic-field detecting element 12 and the planned line 7 is measured. Thus the distance can be detected as the horizontal displacement of the tunnel excavating machine with respect to the planned line 7.