A pair of rotary laser devices are vertically arranged while aligning their respective axes into a single axis and keeping said rotary laser devices away from each other in an axial direction thereof by a predetermined interval, and a light-receiving sensor is kept radially away from the single axis. A plurality of fan-shaped laser beams are emitted from each of the rotary laser devices, while rotating the laser beams. The laser beams include at least one fan-shaped laser beam in a plane inclined to a plane containing said single axis. The laser beams are received and detected with the light-receiving sensor. Inclinations of the rotary laser devices are determined relative to the light-receiving sensor, respectively, based on time intervals of said detection of the laser beams, and the position of the sensor are determined relative to said pair of the rotary laser devices in the axial direction.