PURPOSE: To execute with a high accuracy the marking of a semiconductor wafer, a glass plate, etc., by condensing a laser beam which has transmitted through an object to be worked, to the reverse side of the object to be worked, and also, to the surface, too.

CONSTITUTION: In a laser beam 3a which has been emitted from a laser oscillator 1, a laser beam 3b which has transmitted through an object to be worked 2 is led to a concave reflecting mirror 6 which has been placed at the back of the object to be worked, and its reflected light is condensed to the reverse side of the object to be worked 2. On the other hand, by using an observation use optical device 7, marking is executed with a high accuracy by observing a point Q of the surface of the object to be worked 2. Also, on the upper side of the object to be worked 2, a convex lens 8 of a comparatively small diameter is provided, also a convex lens 9 and a plane mirror 10 are provided on the lower side of the object to be worked 2, and a mark is given simultaneously to the surface and the reverse side of the object to be worked 2. In this way, marking can be executed efficiently.