A target having a reflective part for a laser beam is located at the measuring object position of a bridge girder in such a way that the reflective part includes with respect to the displacement measuring direction and a laser rangefinder is located at the pier. A laser beam projected from the laser rangefinder along the longitudinal direction of the bridge girder impinges on the reflective part and the reflected light of the laser beam returned from the reflective part in the direction generally parallel with the incident direction is received by the laser rangefinder so as to detect variation in the distance from the laser rangefinder to the laser beam reflecting position of the reflective part. Displacement of the bridge girder in the displacement measuring direction is measured utilizing correlation between the detected variation and the displacement of the measuring object position in the displacement measuring direction.