To simplify the measurement of a gap, and eliminate the dispersion due to individuals for obtaining high reliability by making a flexible mechanism of a gap-measuring sensor of remote operation which is arranged to a plurality of measuring points.

The measuring output of a gap measuring sensor 2 is sent to a measuring monitor 10 through a transmitting system that is wired inside a flexible arm 1. This measuring monitor 10 displays in monitor the change in the electric resistance measured by the gap measuring sensor 2. While, on the other hand, a fiberscope 11 as an optical system having an image pickup action for confirming the arrangement position of the gap measuring sensor 2 is detachably fitted to the flexible arm 1 by means of fitting members 12. The fiberscope 11 has a flexible structure and transmits the pickup image in the vicinity of the gap measuring sensor 2, and a monitor 13 for confirming the position is connected to the other end thereof.