The invention provides a method for processing a transparent material, which is applicable to universal materials and by which the propagation loss of an optical function device produced by changing the refractive index is slight. Femtosecond laser pulses are condensed by a lens, and a glass material being fused quartz is relatively moved, wherein a waveguide is formed by forming a continuous high refractive index area. Prior art optical pulses depicted in FIG. 1(a) are one-by-one femtosecond laser pulses at a pulse interval which is slightly shorter or longer than a thermal diffusion time (1 .mu.s) of a substance. Optical pulses according to the invention, which are depicted in FIG. 1(b) are irradiated while using, as a set, two femtosecond laser pulses with a pulse interval which is around the electron lattice energy relaxation time (1 through 100 ps) of a substance.