PURPOSE: To enable non-contact measurement of a gap between a third rail and an upper protective plate to be simply and highly accurately carried out by performing a scanning with a laser beam radiated from a laser distance measuring device toward the third rail from the side of a vehicle to be inspected and in the longitudinal direction sequentially.

CONSTITUTION: A laser distance measuring device 21 mounted on a vehicle 20 to be inspected radiates a laser beam B to a subject in a pulse manner and a distance to the subject is measured from time spent until the reflected beam is received. A rotary mirror 22a reflects the beam B rotating from the distance measuring device 21 to turn the beam to the third rail 1 from the side of the vehicle 20 to be measured while a scanning is performed vertically by the beams B. An arithmetic device 24 calculates a gap L between the rail 1 and the protective plate 6 based on distances to an upper protective plate 5 and rear side protective plate 6 and the rail 1 from the distance measuring device 21 as measures with the distance measuring device 21 while a gap G between the rail 1 and the protective plate 5 is calculated.