To form an internal mark without traces such as a crack, a cutting trace, and an indentation of an outer surface which cause the contamination by focusing a focal point of a transmission laser light on a required internal portion position inside a heat resistant transparent body, and concentrating the energy.

A scanning optical system is arranged at the position of 1/2 thickness of a body 5 of a quartz glass substrate for a photomask with, for example, thickness of 6.5 mm which is manufactured in a required dimension and washed cleanly, in a manner that a center shaft 13b of a second laser light 11b forming a fine focal point of 10 μm diameter is crossed at the angle of 15° against a center shaft 13a of a first laser light 11a adjusted to focus a focal point 14a of the 2 mm diameter. The laser lights 11a, 11b are oscillated and a regular triangle is drawn on the focal point 14a face by the light 11b. Then, the energy density is heightened to dissolve the face by overlaying the lights 11a, 12b so as to form a vivid internal mark 12 which has the regular triangle at the internal position of the body 5 and comprises three lines.