When laser beams with a wavelength of 9.3 .mu.m or 9.6 .mu.m are used, a pulse width t (.mu.sec) which is a radiation time of the laser beam and an energy density E (kw/cm.sup.2) of the laser beam on an X-ray film are set such that they meet requirements based on an area A between line segments A.sub.1 and A.sub.2. Moreover, when laser beams with a wavelength of a 10-micrometer band, such as 10.6 .mu.m, is used, the pulse width and the energy density are set such that they meet requirements based on an area B between line segments B.sub.1 and B.sub.2. As a result, since the pulse width t is within a range of equal to or larger than 3 .mu.sec and smaller than 30 .mu.sec, a high-quality marking pattern with excellent visibility can be formed while improving the productivity of the X-ray film.