PURPOSE: To eliminate invasion of a refuse to an optical path and stay of smoke generated at the time of forming a barn pattern by introducing a laser light to a place out of the path, to be easily operated by a telescopic type reflecting mirror to detect the position of an optical axis.

CONSTITUTION: An He-Ne laser light for a collimation is introduced out of an optical path by two telescopic type reflecting mirrors 4a, 4b disposed axially of the path. The optical axes are detected by targets 6a, 6b. Thus, the positions of two points to be passed with normal laser lights are clarified by the targets 6a, 6b. Here, inclining angles of the mirrors 4a, 4b to the path are not varied. Thus, reflecting angles from the mirrors 4a, 4b become constant, and the positions of the axes positioned by the targets 6a, 6b have reproducibility.