Abstract: The present invention provides a rotary laser beam emitting apparatus capable of being inclined in a direction coinciding with the direction of a reflector. A light emitting unit emits a laser beam, a rotary laser beam projecting unit projects and turns the laser beam emitted by the light emitting unit in a plane, a first photodetector receives at least part of the laser beam projected by the rotary laser beam projecting unit from a predetermined direction, a second photodetector detects a reflected laser beam reflected from a reflector, and a data processing means determines an error in the direction of a main body relative to the reflector on the basis of detection signals provided by the first and the second photodetector.